Remove all RTEMS COFF targets

bfd/
	* config.bfd (*-*-rtemscoff*): Mark as removed.
gas/
	* configure.tgt (sh-*-rtemscoff*): Remove.
ld/
	* configure.tgt (h8300-*-rtemscoff*): Remove.
	(i960-*-rtems*): Likewise.
	(m68*-*-rtemscoff*): Likewise.
	(sh-*-rtemscoff*): Likewise.