ext2: do not register write_super within VFS

Jan Kara removed 'sb->s_dirt' VFS flag references, so we do not need to
register the ext2 'ext2_write_super()' method in the VFS superblock operations,
because 'sb->s_dirt' won't be ever set to 1 and VFS won't ever call
'->write_super()' anyway. Thus, remove the method.

Tested using xfstests.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>