Remove support for cl_half

This removes support for cl_half (typedef'd to half_).

The issue is that the cl_half type is indistinguishable
from the cl_ushort type (both are typedefs for uint16_t)
which caused the cl_khr_fp16 pragma to be injected into
kernels using cl_ushort which causes errors on platforms
that do not support the cl_khr_fp16 extension.