fscrypt: remove ->is_encrypted()

Now that all callers of fscrypt_operations.is_encrypted() have been
switched to IS_ENCRYPTED(), remove ->is_encrypted().

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>