Revert D119669 "[NVPTX] Prefix "$L__" for branch label names"

This reverts commit cccef321.

Broke clang-cuda-t4

```
/buildbot/cuda-t4-0/work/clang-cuda-t4/clang/bin/clang++  -DNDEBUG  -O3 -DNDEBUG   -w -Werror=date-time -UNDEBUG --cuda-path=/buildbot/cuda-t4-0/work/clang-cuda-t4/external/cuda/cuda-11.0 -I/buildbot/cuda-t4-0/work/clang-cuda-t4/external/cuda/cuda-11.0/include --cuda-gpu-arch=sm_75 -std=c++20 -stdlib=libstdc++ --gcc-toolchain=/buildbot/cuda-t4-0/work/clang-cuda-t4/external/cuda/gcc-8 -DSTDLIB_VERSION=2014 -MD -MT External/CUDA/CMakeFiles/complex-cuda-11.0-c++20-libstdc++-8.dir/complex.cu.o -MF External/CUDA/CMakeFiles/complex-cuda-11.0-c++20-libstdc++-8.dir/complex.cu.o.d -o External/CUDA/CMakeFiles/complex-cuda-11.0-c++20-libstdc++-8.dir/complex.cu.o -c /buildbot/cuda-t4-0/work/clang-cuda-t4/llvm-test-suite/External/CUDA/complex.cu
ptxas /tmp/complex-cfa050/complex-sm_75.s, line 250; fatal   : Parsing error near '$L__BB6_2': syntax error
ptxas fatal   : Ptx assembly aborted due to errors
```