[libspirv] delete mix with vector type x and scalar type a (#20399)

SPIR-V OpenCL.ExtendedInstructionSet.100 requires all of the operands
must be the same type for mix.

Author: wenju-he

libclc/libspirv/lib/generic/common/mix.inc

@@ -11,11 +11,3 @@ _CLC_OVERLOAD _CLC_DEF __CLC_GENTYPE __spirv_ocl_mix(__CLC_GENTYPE x,
                                                      __CLC_GENTYPE a) {
   return __clc_mad(y - x, a, x);
 }
-
-#ifndef __CLC_SCALAR
-_CLC_OVERLOAD _CLC_DEF __CLC_GENTYPE __spirv_ocl_mix(__CLC_GENTYPE x,
-                                                     __CLC_GENTYPE y,
-                                                     __CLC_SCALAR_GENTYPE a) {
-  return __spirv_ocl_mix(x, y, (__CLC_GENTYPE)a);
-}
-#endif