[gardening] Remove multiple_timer_test from isolate stress test suite.

The test makes assumptions about timing making it unsuitable for isolate
stress testing bot.

Fixes #61592
TEST=ci

Change-Id: I1537fd4f12a8ff834c05f4e5defffd48f2009e42
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/451800
Reviewed-by: Alexander Markov <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

runtime/tests/concurrency/stress_test_list.json

@@ -3166,7 +3166,6 @@
   "../../../tests/lib/async/intercept_schedule_microtask4_test.dart",
   "../../../tests/lib/async/intercept_schedule_microtask5_test.dart",
   "../../../tests/lib/async/intercept_schedule_microtask6_test.dart",
-  "../../../tests/lib/async/multiple_timer_test.dart",
   "../../../tests/lib/async/null_future_zone_test.dart",
   "../../../tests/lib/async/periodic_timer2_test.dart",
   "../../../tests/lib/async/periodic_timer3_test.dart",