[gardening] Remove deleted test from stress test configuration.

TEST=vm/dart/isolates/concurrency_stress_sanity_test

Change-Id: I3648e58dd301578b44f2dfd5981e892dd377985b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/409184
Commit-Queue: Siva Annamalai <[email protected]>
Reviewed-by: Ryan Macnak <[email protected]>

Author: a-siva

runtime/tests/concurrency/stress_test_list.json

@@ -2100,7 +2100,6 @@
   "../../../tests/language/named_arguments_anywhere/order_side_effects_ok_test.dart",
   "../../../tests/language/new/create_unresolved_type_runtime_test.dart",
   "../../../tests/language/new/expression_type_args_runtime_test.dart",
-  "../../../tests/language/new/prefix_runtime_test.dart",
   "../../../tests/language/new/statement_test.dart",
   "../../../tests/language/nnbd/boolean_conversion/boolean_conversion_strong_test.dart",
   "../../../tests/language/nnbd/const/potentially_constant_types_test.dart",