libceph: Remove unused ceph_crypto_key_encode

ceph_crypto_key_encode() was added in 2010's commit
8b6e4f2d ("ceph: aes crypto and base64 encode/decode helpers")

but has remained unused (the decode is used).

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>