Remove `XLA_CUDA` and other CUDA build flags. (#9582)
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
