trans_free: drop bad assertion

I assumed that trans_free() would always be called before freeing the FSM. But
the actual conn free dance that tries to make sure a release is triggered from
all directions actually may run into a situation where conn->fi is NULL.

The situation is described in OS#3125.

For now simply drop the assert.

The subscr conn and FSM dealloc will soon be glued firmly together; but I want
to add a test against OS#3062 before that, and that would also hit above assertion.

Related: OS#3125 OS#3062
Change-Id: I5c30e0f9545fb76615776ff6cc16b56aeb5b043a

src/libmsc/transaction.c

@@ -158,9 +158,6 @@ void trans_free(struct gsm_trans *trans)
 	trans->conn = NULL;
 	talloc_free(trans);
 
-	/* trans_free() should always happen while the fi is still around. */
-	OSMO_ASSERT(conn->fi);
-
 	/* Possibly this was the last transaction used by this conn. */
 	subscr_conn_release_when_unused(conn);
 }