ext4: Update EXT4_USE_FOR_EXT2 description

Configuration option EXT4_USE_FOR_EXT2 has no effect on ext3 support.
Support for ext3 is always included now.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Fixes: c290ea01 ("fs: Remove ext3 filesystem driver")
Cc: Jan Kara <jack@suse.cz>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Signed-off-by: Jan Kara <jack@suse.com>