[OpenCL] Remove decls for scalar vloada_half and vstorea_half* fns

These functions are not part of the OpenCL C specification.

See https://github.com/KhronosGroup/OpenCL-Docs/issues/648 for a
clarification regarding the vloada_half declarations.

Reviewed By: Anastasia

Differential Revision: https://reviews.llvm.org/D108761