elf: Remove R_RISCV_[GT]PREL_[IS] relocation defines

These were internal binutils relocations that have no way to be
generated in assembly nor will ever be seen in the output, and so should
never have been defined in the psABI in the first place. They have
therefore been removed from the spec as of [1], so do so here too.

[1] https://github.com/riscv/riscv-elf-psabi-doc/commit/44f98e0fd8104def00f2a5a8d94b23dd647d18fb