[ELF] Delete R_PPC64_CALL_PLT from isRelExpr()

It was added by D46654 but is actually never used.
R_PPC64_CALL_PLT (was: R_PPC_CALL_PLT) is a static link-time constant.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D62994

llvm-svn: 362788