Revert "mtd: move mxcnd_remove to .exit.text"

This reverts commit 82613b0d, because
commit daa0f15a

 (mtd: don't use __exit_p to wrap mxcnd_remove) is a
better solution. Not having a remove callback breaks rebinding because
resources are not freed on remove.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>