[clang] clang/HIP: Remove some unused ocml function declarations (PR #204735)

[Kewen Meng via cfe-commits]

Kewen12 wrote:

Hello! Some of the removed the functions are still used by amd_hip_fb16.h (https://github.com/ROCm/clr/blob/amd-staging/hipamd/include/hip/amd_detail/amd_hip_fp16.h) e.g `__ocml_sin_2f16(x), __ocml_cos_2f16(x), __ocml_sqrt_2f16(x)` which our downstream HIP testing depends on. 

Do we have plan to update amd_hip_fp16.h accordingly, what is the best way to mitigate this? Thanks! 

cc: @jplehr 

https://github.com/llvm/llvm-project/pull/204735