Remove GnuHashTableSection::getShift2().

Choosing a Shift2 value based on wordsize is cargo-culted from gold.
Assuming that djb hash is a good hash function, choosing bits [4,9]
shouldn't be any worse or better than choosing bits [5,10]. We shouldn't
have copied that behavior that we can't justify in the first place.

Differential Revision: https://reviews.llvm.org/D44547

llvm-svn: 327921