Remove workaround on `AbsFOpConversion` with `bf16`

[whitneywhtsang]

Since LLVM now support bfloat16, llvm.fabs.bf16 (https://llvm.org/docs/LangRef.html#llvm-fabs-intrinsic) should be supported by SPIRV-LLVM-Translator.
However, SPIRV-LLVM-Translator fails with the following error when compiling llvm.fabs.bf16 (kernel.ll.txt).

InvalidFunctionCall: Unexpected llvm intrinsic:
 llvm.fabs.bf16 [Src: /home/jovyan/SPIRV-LLVM-Translator/lib/SPIRV/SPIRVUtil.cpp:1883 false ]

#1228 worked around it by bitcast-ing to and from i16 when performing AbsFOpConversion.

This issue is to track removal of the workaround when SPIRV-LLVM-Translator adds the support.

[whitneywhtsang]

Update: it is not yet implemented in SPIRV-LLVM-Translator.

[whitneywhtsang]

Not yet implemented, checked with spirv translator KhronosGroup/SPIRV-LLVM-Translator@37a91c7 and driver igc-19472:
https://github.com/intel/intel-xpu-backend-for-triton/actions/runs/15717920447/job/44294235206