Simplify build by removing GC tweaking logic

Mono 3.0.x was the first version when Mono's SGEN GC could be
considered stable, and F# was using it by default for this
version.

But then Mono 3.2.0 was released, which defaults to SGEN GC
anyway, so there's no need to tweak F# build or scripts anymore,
as we can simplify all this by just making F# use what the
underlying mono uses as default.

If some user is interested in using SGEN GC, she should just
make sure that she's using Mono 3.2.x or higher, or just adjust
MONO_ENV_OPTIONS env var herself.