Reverts "Add UI Benchmarks (#143542)" (#143798)

Reverts flutter/flutter#143542

Initiated by: goderbauer

Reason for reverting: Failing post-submit, see https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8755531866181499153/+/u/run_microbenchmarks/stdout

Original PR Author: bernaferrari

Reviewed By: {goderbauer}

This change reverts the following previous change:
Original Description:
Part 1 of flutter/flutter#138481

Cherry picking @ignatz work.

Author: auto-submit[bot]

dev/benchmarks/microbenchmarks/lib/foundation/observer_list_bench.dart

@@ -57,7 +57,6 @@ TaskFunction createMicrobenchmarkTask({
       ...await runMicrobench('lib/foundation/all_elements_bench.dart'),
       ...await runMicrobench('lib/foundation/change_notifier_bench.dart'),
       ...await runMicrobench('lib/foundation/clamp.dart'),
-      ...await runMicrobench('lib/foundation/observer_list_benchmark.dart'),
       ...await runMicrobench('lib/foundation/platform_asset_bundle.dart'),
       ...await runMicrobench('lib/foundation/standard_message_codec_bench.dart'),
       ...await runMicrobench('lib/foundation/standard_method_codec_bench.dart'),