[GC] Remove so called PreCall safepoints

Remove another bit of unused configuration potential from GCStrategy.  It's not entirely clear what the intention here was, but from the docs, it sounds like this may have been subsumed by patchable call support.

Note: This change is deliberately small to make it clear that while implemented, there's nothing using the option.  A following NFC will do most of the simplifications.
llvm-svn: 346701