ext4: remove redundant condition check

ext4_xattr_destroy_cache() can handle NULL pointer correctly,
so there is no need to check NULL pointer before calling
ext4_xattr_destroy_cache().

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>