Remove XLA_CUDA and other CUDA build flags.

[ysiraichi]

This PR removes the uses and mentions to XLA_CUDA and TF_CUDA_COMPUTE_CAPABILITIES flags. They are related to the now deprecated CUDA build. This PR also removes

Key Changes:

• (.bazelrc) Removed CUDA bazel configuration
• (build_util.py) Removed the translation of XLA_CUDA environment variable to --config=cuda bazel argument
• Removed uses of XLA_CUDA and TF_CUDA_COMPUTE_CAPABILITIES throughout the codebase
• Removed some logic for compiling PyTorch/XLA with CUDA support

[zhanyong-wan]

Nice!