opcodes/riscv-dis.c: Remove last_map_state

Before changing the core disassembler, we take care of minor code clarity
issues and improve readability.

This commit removes unused variable last_map_state (set by the
print_insn_riscv function but not read anywhere else).

opcodes/ChangeLog:

	* riscv-dis.c (last_map_state): Remove.
	(print_insn_riscv): Remove setting last_map_state.