Remove incomplete support for plain MD5 from OCF.

Although a few drivers supported this algorithm, there were never any
in-kernel consumers. cryptosoft and cryptodev never supported it,
and there was not a software xform auth_hash for it.

Reviewed by: cem
Relnotes: yes
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D24767