[RISCV][ELF] Remove internal binutils relocations

These relocations have been removed from the psABI doc in
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/199. This
patch simply updates llvm/include/llvm/BinaryFormat/ELFRelocs/RISCV.def
to match.

Differential Revision: https://reviews.llvm.org/D112999