[ARM] remove reference to non-existent MTD_OBSOLETE_CHIPS

The CONFIG variable MTD_OBSOLETE_CHIPS was deleted in commit
ba7cc09c.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>