Remove TODO on embedder a11y unit tests (flutter#13346)

Removes a TODO implying that the embedder accessibility unittests were
disabled due to flakiness.

The tests were originally disabled in
flutter/engine#9482.

The issue tracking re-enabling the tests was
flutter#35218.

This issue was resolved, and the tests re-enabled, in
flutter/engine#9585.

shell/platform/embedder/tests/embedder_a11y_unittests.cc

@@ -21,9 +21,6 @@ namespace testing {
 
 using Embedder11yTest = testing::EmbedderTest;
 
-// TODO: This test has been disabled as it is flaky (more reproducible in
-// profile more). Multiple calls to a11y changed handler in Dart code is
-// suspected. https://github.com/flutter/flutter/issues/35218
 TEST_F(Embedder11yTest, A11yTreeIsConsistent) {
   auto& context = GetEmbedderContext();