Origin/remove redundant gc collect

[rarepops]

During unload_thread_main, we call mono_gc_collect 3 times.

First: https://github.com/mono/mono/blob/280e9d2423549d86686716f0818bcdbac9702ea1/mono/metadata/gc.c#L455

Second: https://github.com/mono/mono/blob/b4c506c3045516349d03ce8f3fe9fa5b79a7271c/mono/metadata/appdomain.c#L3305

Third: https://github.com/mono/mono/blob/b4c506c3045516349d03ce8f3fe9fa5b79a7271c/mono/metadata/appdomain.c#L3324

This PR #ifdefs the Second GC Collect because Boehm does not use remsets, and the comment explaining it mentions that

We need to make sure that we don't have any remsets pointing into static data...(cont)
However, Unity uses Boehm so it can be defined out.

There seems to be no good reason for the third GC collect, as it is already called before in mono_domain_finalize

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[joncham]

Looks okay to me. Might be worth trying to land upstream PR and se feedback/test results there before merging here?