Delete RegisterAndUnregister_Parallel cancellation test (#1691)

We've decided this isn't a relevant scenario to track.  The type was originally optimized for this scenario at the expense of serial usage, but since then the majority use case is the serial one.

Author: stephentoub

src/benchmarks/micro/libraries/System.Threading/Perf.CancellationToken.cs

@@ -16,11 +16,6 @@ public class Perf_CancellationToken
         [Benchmark]
         public void RegisterAndUnregister_Serial() => _token.Register(() => { }).Dispose();
 
-        [Benchmark(OperationsPerInvoke = 1_000_000)]
-        [BenchmarkCategory(Categories.NoWASM)]
-        public void RegisterAndUnregister_Parallel() =>
-            Parallel.For(0, 1_000_000, i => _token.Register(() => { }).Dispose());
-
         [Benchmark]
         public void Cancel()
         {