Revert "ipsec: Drain async ipsec_offload work when destroying a vnet"

This change can cause a deadlock in some cases, since it's possible for
VNET teardown to happen in the context of taskqueue_thread, and
ipsec_accel_sync() drains taskqueue_thread's work queue.

This reverts commit e196b12f4d4d13982d42cf7841b77dca1405effa.