Don't handle BFD_RELOC_16 in XCOFF reloc_type_lookup

It's not needed for sizing fixups since 0e2779e9, and wrong to emit
this reloc to the object file.

	* coff-rs6000.c (_bfd_xcoff_reloc_type_lookup): Remove BFD_RELOC_16.
	* coff64-rs6000.c (xcoff64_reloc_type_lookup): Likewise.