Revert "[DEBUGINFO, NVPTX] Enable support for the debug info on NVPTX target."

This reverts commit r351846.

This patch may generate illegal assembly code, see

```
$ ./bin/clang -cc1 -triple nvptx64-nvidia-cuda -aux-triple x86_64-grtev4-linux-gnu -S -disable-free -disable-llvm-verifier -discard-value-names -main-file-name new.cc -mrelocation-model pic -pic-level 2 -mthread-model posix -fmerge-all-constants -mdisable-fp-elim -relaxed-aliasing -no-integrated-as -mpie-copy-relocations -munwind-tables -fcuda-is-device -target-feature +ptx60 -target-cpu sm_35 -dwarf-column-info -debug-info-kind=line-directives-only -dwarf-version=2 -debugger-tuning=gdb -o empty.s -x cuda empty.cc
$  cat empty.s
//
// Generated by LLVM NVPTX Back-End
//

.version 6.0
.target sm_35
.address_size 64

	}
```

llvm-svn: 351966