Simplify IPsec transform-specific teardown.

• Rename from the teardown callback from 'zeroize' to 'cleanup' since this no longer zeroes keys.

• Change the callback return type to void. Nothing checked the return value and it was always zero.

• Don't have esp call into ah since it no longer needs to depend on this to clear the auth key. Instead, both are now private and self-contained.

Reviewed by: delphij
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D25443