PSA: Intent to remove XLA:GPU legacy emitters by the end of October.

[jreiffers]

If you're using Nvidia GPUs and not using the flag xla_gpu_mlir_emitter_level,
you can stop reading.

Hey everyone, the new XLA:GPU emitters have been fully launched for over two
months now. Legacy emitters are increasingly bit rotting and falling back in
performance. There is no test coverage for them, so they're essentially
unmaintained.

We intend to remove legacy emitters from the code base by the end of October.
If you have a use case that requires them, we can extend the deadline. However,
this code will be deleted eventually, so please plan accordingly.

Please let me know if you have any questions.