Reverts "Reland Add UI Benchmarks (#143542) (#143799)" (#143946)

Reverts flutter/flutter#143799

Initiated by: goderbauer

Reason for reverting: The microbenchmark failed two run two times in a row after this was committed (see https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8755379171959141089/+/u/run_microbenchmarks/stdout).

Original PR Author: bernaferrari

Reviewed By: {goderbauer}

This change reverts the following previous change:
Original Description:
Re-lands flutter/flutter#143542 (which is part 1 of flutter/flutter#138481)

The filename was wrong 🫠

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_bench.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'),