Revert "kill stale oc for backend synth"

This reverts commit de3e581e.

tests/saintmode/test04.vtc from varnish-modules revealed that there may
be use cases where it is implied that vcl_backend_error will leave a
ttl=0 object and the sale object will remain in place.

This can be resolved by:

	- return(abandon) for these cases or
	- making the HSH_Kill conditional on bo->fetch_objcore->ttl > 0

but I already has concerns regarding the impact of this change and have
them even more now.

We need time to ponder about the consequences and a documented
inefficiency is better than unexpected behaviour in a release.

bin/varnishd/cache/cache_fetch.c

@@ -776,8 +776,6 @@ vbf_stp_error(struct worker *wrk, struct busyobj *bo)
 	VSB_destroy(&synth_body);
 	HSH_Unbusy(wrk, bo->fetch_objcore);
 	ObjSetState(wrk, bo->fetch_objcore, BOS_FINISHED);
-	if (bo->stale_oc != NULL)
-		HSH_Kill(bo->stale_oc);
 	return (F_STP_DONE);
 }
 

bin/varnishtest/tests/r02946.vtc

-varnishtest "#2946 - objcore leak for backend_synth"
-
-varnish v1 -vcl {
-	backend bad { .host = "${bad_backend}"; }
-
-	sub vcl_backend_error {
-		if (bereq.http.abandon) {
-			return (abandon);
-		}
-		set beresp.status = 200;
-		set beresp.ttl = 0.0001s;
-		set beresp.grace = 1h;
-	}
-} -start
-
-client c1 -repeat 20 -keepalive {
-	txreq
-	rxresp
-	expect resp.status == 200
-
-	delay 0.001
-} -run
-
-delay 2
-
-client c1 {
-	txreq -hdr "abandon: true"
-	rxresp
-	expect resp.status == 200
-	expect resp.http.age > 1
-} -run
-
-varnish v1 -expect n_objectcore == 1