Revert "always adds alert label for alert dialog in Android"

[jonahwilliams]

Reverts #65973

Breaks android semantics integration test:

2020-09-16T18:49:33.953383: stdout: [+11309 ms] 00:32 �[32m+8�[0m: AccessibilityBridge Popup Controls Modal alert dialog has correct Android semantics�[0m
2020-09-16T18:49:35.841076: stdout: [+1887 ms] 00:34 �[32m+8�[0m�[31m -1�[0m: AccessibilityBridge Popup Controls Modal alert dialog has correct Android semantics �[1m�[31m[E]�[0m�[0m
2020-09-16T18:49:35.841274: stdout: [        ]   Expected: AndroidSemanticsNode with className: android.view.View with actions: [AndroidSemanticsAction.accessibilityFocus] with flag isChecked: false with flag isEnabled: true with flag isFocusable: true
2020-09-16T18:49:35.841512: stdout: [        ]     Actual: AndroidSemanticsNode:<{flags: {isEditable: false, isEnabled: true, isDismissable: false, isCheckable: false, isChecked: false, isFocused: false, isLongClickable: false, isFocusable: true, isPassword: false}, id: 94, text: null, rect: {height: 69, width: 696, bottom: 258.6666564941406, top: 235.6666717529297, left: 64.0, right: 296.0}, actions: [128], className: android.view.View, liveRegion: 0, contentDescription: Title text}>
2020-09-16T18:49:35.841761: stdout: [        ]      Which: Expected actions: [AndroidSemanticsAction.accessibilityFocus]
2020-09-16T18:49:35.842043: stdout: [        ]             Actual actions: [AndroidSemanticsAction.clearAccessibilityFocus]
2020-09-16T18:49:35.842284: stdout: [        ]             Unexpected: {AndroidSemanticsAction.clearAccessibilityFocus}
2020-09-16T18:49:35.842473: stdout: [        ]             Missing: {AndroidSemanticsAction.accessibilityFocus}
2020-09-16T18:49:35.842720: stdout: [        ]   Alert Title button doesn't have the right semantics
2020-09-16T18:49:35.867026: stdout: [  +24 ms]   package:test_api/src/frontend/expect.dart 155:31               fail
2020-09-16T18:49:35.867608: stdout: [        ]   package:test_api/src/frontend/expect.dart 150:3                _expect
stdout: [        ]   package:test_api/src/frontend/expect.dart 59:3                 expect
stdout: [        ]   test_driver/main_test.dart 607:13                              main.<fn>.<fn>.<fn>
2020-09-16T18:49:35.868270: stdout: [        ]   ===== asynchronous gap ===========================
stdout: [        ]   dart:async/zone.dart 1129:19                                   _CustomZone.registerBinaryCallback
stdout: [        ]   dart:async-patch/async_patch.dart 94:8                         _asyncErrorWrapperHelper
2020-09-16T18:49:35.868676: stdout: [        ]   package:test_api/src/backend/invoker.dart                      Invoker.waitForOutstandingCallbacks.<fn>
stdout: [        ]   dart:async/zone.dart 1190:13                                   _rootRun
2020-09-16T18:49:35.869146: stdout: [        ]   dart:async/zone.dart 1093:19                                   _CustomZone.run
stdout: [        ]   dart:async/zone.dart 1630:10                                   _runZoned
2020-09-16T18:49:35.869698: stdout: [        ]   dart:async/zone.dart 1550:10                                   runZoned
stdout: [        ]   package:test_api/src/backend/invoker.dart 228:5                Invoker.waitForOutstandingCallbacks

FYI @chunhtai

[jonahwilliams]

Landing TBR to help tree green-ness

[jonahwilliams]

Reverting this also fixed flutter_gallery_ios__transition_perf, though it isn't clear from the logs if that is due to a flake or another cause