[CodeGen] Remove experimental deferred spilling from GreedyRegAlloc (#137850)

This experimental option was introduced in 2015 via commit 1192294, and
the target hook was added in 2020 via commit 99e865b6. There does not
appear to have ever been a use of this target hook in tree.

This code is complicating one of the most complicated and hard to
understand parts of our code base, and was an experiment introduced
nearly 10 years ago. Let's get rid of it.

Note that the idea described in the original patch is not neccessarily a
bad one, and we might return to it someday.

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