Fix Travis breakage caused due to unimported idx module

Summary:
**Summary**
I broke the CI build by using `idx` in the `DraftTreeAdapter` file, which is a FB-internal module not available on this project

Reviewed By: jjs0

Differential Revision: D9557731

Pulled By: jjs0

fbshipit-source-id: 1edaee3ac51525afb276a29462ffd29518b18c29

Author: niveditc

src/component/utils/exploration/DraftTreeAdapter.js

@@ -17,7 +17,6 @@ import type {RawDraftContentBlock} from 'RawDraftContentBlock';
 import type {RawDraftContentState} from 'RawDraftContentState';
 
 const generateRandomKey = require('generateRandomKey');
-const idx = require('idx');
 const invariant = require('invariant');
 
 const traverseInDepthOrder = (
@@ -127,7 +126,7 @@ const DraftTreeAdapter = {
 
       // nesting
       if (depth > 0) {
-        let parent = idx(lastListDepthCacheRef, _ => _[depth - 1]);
+        let parent = lastListDepthCacheRef[depth - 1];
         if (parent == null) {
           parent = {
             key: generateRandomKey(),