Remove a racy assertion on the OC_EF_DYING state

This state could be set by the expiry timer even though a reference is
grabbed during lookup, causing the assertion to trigger.

Fixes: #1539

bin/varnishd/cache/cache_req_fsm.c

@@ -358,7 +358,6 @@ cnt_lookup(struct worker *wrk, struct req *req)
 	case VCL_RET_DELIVER:
 		if (boc != NULL) {
 			AZ(oc->flags & OC_F_PASS);
-			AZ(oc->exp_flags & OC_EF_DYING);
 			AZ(boc->busyobj);
 			VBF_Fetch(wrk, req, boc, oc, VBF_BACKGROUND);
 		} else {