RISC-V: Pc-rel to gp-rel relaxation function cleanup.

	bfd/
	* elfnn-riscv.c (riscv_init_pcgp_relocs): Add explanatory comment.
	(riscv_free_pcgp_relocs, riscv_record_pcgp_reloc): Likewise.
	(riscv_find_pcgp_hi_reloc, riscv_reocrd_pcgp_lo_reloc): Likewise.
	(riscv_find_pcgp_lo_reloc): Likewise.
	(riscv_delete_pcgp_hi_reloc, riscv_use_pcgp_hi_reloc): Delete.
	(riscv_delete_pcgp_lo_reloc): Likewise.
	(_bfd_riscv_relax_pc): Don't call riscv_use_pcgp_hi_reloc. Replace
	calls to riscv_delete_pcgp_lo_reloc and riscv_delete_pcgp_hi_reloc
	with TRUE.  Mark abfd arg as ATTRIBUTE_UNUSED.