Fix the test. (#20701)

Fix the test. (#20701)

As https://docs.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.gchandle.free?view=netframework-4.7.2 says "The caller must ensure that for a given handle, Free is called only once.".

So delete the second call to `Free()`.

I believe it changes the desired behaviour for this test but it was added before 2010 (via source depo) and there are no comments about the regression that it was able to repro, so it is not worth to investigate that.