Remove OpenXLA CUDA fallback and _XLAC_cuda_functions.so extension.

[ysiraichi]

This PR removes the OpenXLA CUDA fallback implementation, and also the _XLA_cuda_functions.so extension, completely. Starting on this PR, the fallback shall be run only on CPU.

Key Changes:

• Remove aten_cuda_functions.cpp and aten_cuda_functions.h
• Remove the OpenXLA CUDA fallback functions from aten_fallback.cpp
• Remove the _XLAC_cuda_functions.so library from BUILD
• Remove the Python _XLAC_cuda_functions.so extension from setup.py
• Remove the conditional loading of _XLAC_cuda_functions.so Python extension from torch_xla/init.py

[zhanyong-wan]

Great! Thank you!