OCF: Remove support for asymmetric cryptographic operations.

There haven't been any non-obscure drivers that supported this
functionality and it has been impossible to test to ensure that it
still works. The only known consumer of this interface was the engine
in OpenSSL < 1.1. Modern OpenSSL versions do not include support for
this interface as it was not well-documented.

Reviewed by: cem
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D29736