ext4: remove "nokey" check from ext4_lookup()

Now that fscrypt_has_permitted_context() correctly handles the case
where we have the key for the parent directory but not the child, we
don't need to try to work around this in ext4_lookup().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>