Removes gtest config from //testing/buildbot.

This configuration got moved to the chromium_android.py config in the chromium_tests recipe module, since that's where this config seems to be for the chromium.linux Android Tests (dbg) builder.

base_unittests (along with several other unit tests) required --isolate-file-path <isolate-file> to know what to push to the device. We could either revise //testing/scripts/android_gtest_test.py (which is used for exactly one suite on the main Android bots), use gtest_tests and adapt the test generator on the recipe_module side to do what we need it to, or do what the main Android bots do.

We've opted for the latter with this CL & the matching one on the infra side, but I don't have a strong opinion one way or another.

BUG=

Review URL: https://codereview.chromium.org/1405253008

Cr-Commit-Position: refs/heads/master@{#357017}

testing/buildbot/chromium.android.json

@@ -4,13 +4,7 @@
       "compositor_unittests_apk"
     ]
   },
-  "Android Aura Tester (dbg)": {
-    "gtest_tests": [
-      {
-        "test": "base_unittests"
-      }
-    ]
-  },
+  "Android Aura Tester (dbg)": {},
   "Android GN Builder (dbg)": {
     "additional_compile_targets": [
       "blimp_tests",