Fix PR 110085: `make clean` in GCC directory on sh target causes a failure

On sh target, there is a MULTILIB_DIRNAMES (or is it MULTILIB_OPTIONS) named m2,
this conflicts with the langauge m2. So when you do a `make clean`, it will remove
the m2 directory and then a build will fail. Now since r0-78222-gfa9585134f6f58,
the multilib directories are no longer created in the gcc directory as libgcc
was moved to the toplevel. So we can remove the part of clean that removes those
directories.

Tested on x86_64-linux-gnu and a cross to sh-elf that `make clean` followed by
`make` works again.

OK?

gcc/ChangeLog:

	PR bootstrap/110085
	* Makefile.in (clean): Remove the removing of
	MULTILIB_DIR/MULTILIB_OPTIONS directories.