Remove unittests from windows_host_engine GN targets. (#168991)

Closes flutter/flutter#168990.

Will shave a few minutes off builds, particularly in resource sensitive
environments like releases.

Author: matanlurey

engine/src/flutter/ci/builders/windows_host_engine.json

@@ -49,7 +49,6 @@
             "ninja": {
                 "config": "ci/host_debug",
                 "targets": [
-                    "flutter:unittests",
                     "flutter/build/archives:artifacts",
                     "flutter/build/archives:embedder",
                     "flutter/tools/font_subset",