md: remove redundant variable q

The pointer q is assigned but never read; it is redundant and can
be removed.  Cleans up clang warning:

drivers/md/md-multipath.c:260:4: warning: Value stored to 'q' is
never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Shaohua Li <shli@fb.com>