RISC-V: Clean up unused variable [NFC]

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_load_store):
	Remove `value`.
	(expand_cond_len_op): Ditto.
	(expand_gather_scatter): Ditto.
	(expand_lanes_load_store): Ditto.
	(expand_fold_extract_last): Ditto.