Simplify IPsec transform-specific teardown.

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