[nvptx] Only support the 'm' inline assembly memory constraint. NFC.

Summary:
NVPTX doesn't seem to support any additional constraints. Therefore remove
the target hook.

No functional change intended.

Reviewers: jholewinski

Reviewed By: jholewinski

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D8209

llvm-svn: 239395