[globalisel] Remove dead code from GlobalISelEmitter

Summary: Found by GCC's -Wunused-function.

Patch by Kim Gräsman

Reviewers: ab, dsanders, llvm-commits

Reviewed By: dsanders

Subscribers: rovka, kristof.beyls

Differential Revision: https://reviews.llvm.org/D50611

llvm-svn: 339528