[mlir][ROCDL] Remove unneeded bf16 expansion in LowerGPUToRocdl

The umbrella pass fol lowering GPU ops to ROCDL (aka  lowering to LLVM
+ the AMDGPU-specific setup) would call the arith patterns that
manually implemented extf and truncf on bfloat because the LLVM AMDGPU
backend used to not suppport those operaitons.

Since the backend does now support these operations and has for quite
some time, remove these patterns from the default lowering flow.

mlir/lib/Conversion/GPUToROCDL/LowerGpuOpsToROCDLOps.cpp

@@ -319,7 +319,6 @@ struct LowerGpuOpsToROCDLOpsPass final
     {
       RewritePatternSet patterns(ctx);
       populateGpuRewritePatterns(patterns);
-      arith::populateExpandBFloat16Patterns(patterns);
       (void)applyPatternsGreedily(m, std::move(patterns));
     }