RISC-V: Cleanup the include/opcode/riscv-opc.h.

The include/opcode/riscv-opc.h file is no longer automatically generated,
so we remove the misleading comments and add new ones.  Besides, the CAUSE_*
macros and DECLARE_CAUSE are unused for binutils and gdb.  Therefore, remove
them, too.

	include/
	* opcode/riscv-opc.h: Cleanup and remove the unused macros.