Delete rx and rl78 md_end

There isn't much point in having functions that do nothing

	* config/tc-rl78.c (rl78_md_end): Delete.
	* config/tc-rl78.h (md_end): Don't define.
	* config/tc-rx.c (rx_md_end): Delete.
	* config/tc-rx.h (md_end): Don't define.