[NFC][libspirv] delete clc_core.h which is now irrelevant (#18092)
The file was intended to share code between SPIR-V and OpenCL. Now
shared code are moving to clc.
Also delete unused files: clc_add_sat.cl, clc_mad_sat.cl, clc_sub_sat.cl
and clc_ldexp.cl.
llvm-diff shows no change to libspirv-nvptx64--nvidiacl.bc and
libspirv-amdgcn--amdhsa.bc.
