[TableGen] Remove unused target intrinsic generation logic

AMDGPU was the last in tree target to use this tablegen mode. I plan to
split up the global intrinsic enum similar to the way that clang
diagnostics are split up today. I don't plan to build on this mode.

Reviewers: arsenm, echristo, efriedma

Reviewed By: echristo

Differential Revision: https://reviews.llvm.org/D71318