[MIPS] Remove vmlinux.rm200 target from makefile.

Long ago in the dark ages this was used a MIPS a.out binary to be used
with Milo which is obsolete since years.

Signed-off-by: Yoichi Yuasa <yoichi_yuasa@tripeaks.co.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>