[Comgr] Do not compute opencl-c.h hash (llvm#1050)

The hash is not used, the header doesn't ever change.

The header should change together with llvm's version (which is taken
into account by the cache). But on several of our builds we pick the
wrong header (from the system) anyways.

Co-authored-by: Juan Manuel Martinez Caamaño <juamarti@amd.com>

Author: jmmartinez

amd/comgr/cmake/opencl_pch.cmake

@@ -50,7 +50,3 @@ endfunction()
 
 generate_pch(1.2)
 generate_pch(2.0)
-
-# hash the opencl header and pass the result to comgr compilation
-file(SHA256 ${OPENCL_C_H} OPENCL_C_SHA)
-list(APPEND AMD_COMGR_PRIVATE_COMPILE_DEFINITIONS "OPENCL_C_SHA=${OPENCL_C_SHA}")