[Synthetics] remove full screenshots (#189911)

## Summary

We have not used full screenshot image data since early alpha versions
of Synthetics (pre 7.14.0), and there is no reason to _not_ use
screenshot blocks instead as they make far more efficient storage
performance.

### Release note
Removes support for Heartbeat browser monitor screenshots for lower
versions of Heartbeat. Please upgrade to Heartbeat 7.17.24 to continue
using Heartbeat browser monitor screenshots.

### Testing 
Screenshots are just for browser monitors, and browser monitors are in
Beta in 7.17.x and will never be moved to GA. I don't believe this
ticket needs additional testing beyond passing integration tests.

Author: dominiqueclarke

x-pack/plugins/uptime/server/rest_api/pings/journey_screenshots.test.ts

@@ -86,73 +86,6 @@ describe('journey screenshot route', () => {
     expect(response.body.screenshotRef).toEqual(mock);
   });
 
-  it('returns full screenshot blob', async () => {
-    const mock = {
-      synthetics: {
-        blob: 'a blob',
-        blob_mime: 'image/jpeg',
-        step: {
-          name: 'a step name',
-        },
-        type: 'step/screenshot',
-      },
-      totalSteps: 3,
-    };
-    const route = createJourneyScreenshotRoute({
-      // @ts-expect-error incomplete implementation for testing
-      requests: {
-        getJourneyScreenshot: jest.fn().mockReturnValue(mock),
-      },
-    });
-
-    // @ts-expect-error incomplete implementation for testing
-    expect(await route.handler(libs)).toMatchInlineSnapshot(`
-      Object {
-        "body": Object {
-          "data": Array [
-            105,
-            185,
-            104,
-          ],
-          "type": "Buffer",
-        },
-        "headers": Object {
-          "cache-control": "max-age=600",
-          "caption-name": "a step name",
-          "content-type": "image/jpeg",
-          "max-steps": "3",
-        },
-        "message": "Ok",
-        "status": 200,
-      }
-    `);
-  });
-
-  it('defaults to png when mime is undefined', async () => {
-    const mock = {
-      synthetics: {
-        blob: 'a blob',
-        step: {
-          name: 'a step name',
-        },
-        type: 'step/screenshot',
-      },
-    };
-    const route = createJourneyScreenshotRoute({
-      // @ts-expect-error incomplete implementation for testing
-      requests: {
-        getJourneyScreenshot: jest.fn().mockReturnValue(mock),
-      },
-    });
-
-    // @ts-expect-error incomplete implementation for testing
-    const response = await route.handler(libs);
-
-    expect(response.status).toBe(200);
-    // @ts-expect-error incomplete implementation for testing
-    expect(response.headers['content-type']).toBe('image/png');
-  });
-
   it('returns 404 for screenshot missing blob', async () => {
     const route = createJourneyScreenshotRoute({
       // @ts-expect-error incomplete implementation for testing

x-pack/plugins/uptime/server/rest_api/pings/journey_screenshots.ts

@@ -6,7 +6,7 @@
  */
 
 import { schema } from '@kbn/config-schema';
-import { isRefResult, isFullScreenshot } from '../../../common/runtime_types/ping/synthetics';
+import { isRefResult } from '../../../common/runtime_types/ping/synthetics';
 import { UMServerLibs } from '../../lib/lib';
 import { ScreenshotReturnTypesUnion } from '../../lib/requests/get_journey_screenshot';
 import { UMRestApiRouteFactory } from '../types';
@@ -37,15 +37,7 @@ export const createJourneyScreenshotRoute: UMRestApiRouteFactory = (libs: UMServ
       stepIndex,
     });
 
-    if (isFullScreenshot(result) && typeof result.synthetics?.blob !== 'undefined') {
-      return response.ok({
-        body: Buffer.from(result.synthetics.blob, 'base64'),
-        headers: {
-          'content-type': result.synthetics.blob_mime || 'image/png', // falls back to 'image/png' for earlier versions of synthetics
-          ...getSharedHeaders(result.synthetics.step.name, result.totalSteps),
-        },
-      });
-    } else if (isRefResult(result)) {
+    if (isRefResult(result)) {
       return response.ok({
         body: {
           screenshotRef: result,