libceph: remove the unused macro AES_KEY_SIZE

This removes the no longer used macro AES_KEY_SIZE as no functions use
this macro anymore and thus this macro can be removed due it no longer
being required.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>