Remove deprecated XLA GPU flags.

docs/gpu_performance_tips.md

@@ -44,11 +44,8 @@ example, we can add this to the top of a Python file:
 ```python
 import os
 os.environ['XLA_FLAGS'] = (
-    '--xla_gpu_enable_triton_softmax_fusion=true '
     '--xla_gpu_triton_gemm_any=True '
-    '--xla_gpu_enable_async_collectives=true '
     '--xla_gpu_enable_latency_hiding_scheduler=true '
-    '--xla_gpu_enable_highest_priority_async_stream=true '
 )
 ```
 
@@ -58,9 +55,6 @@ training on Nvidia GPUs](https://github.com/NVIDIA/JAX-Toolbox/blob/main/rosetta
 
 ### Code generation flags
 
-* **--xla_gpu_enable_triton_softmax_fusion** This flag enables an automatic
-  softmax fusion, based on pattern-matching backed by Triton code generation.
-  The default value is False.
 * **--xla_gpu_triton_gemm_any** Use the Triton-based GEMM (matmul) emitter for
   any GEMM that it supports. The default value is False.