Remove the PTX back-end and all of its artifacts (triple, etc.)

This back-end was deprecated in favor of the NVPTX back-end.

NV_CONTRIB

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157417 91177308-0d34-0410-b5e6-96231b3b80d8
95 files changed