Revert "ikev2: Insert MOBIKE tasks at the front of the queue"

This reverts commit 3293d146289d7c05e6c6089ae1f7cdbcea378e63.

The position of tasks in the queue does not actually determine the order
in which they are activated.  Instead this is determined by the
statements in task_manager_v2_t.initiate().
ikev2: Reorder task activation for established IKE SAs

We now prefer MOBIKE tasks over delete tasks then the rest.