Assert that if a connection is closed before being used, it is so for a reason

... but we can not make any assumption as to why

Fixes #3664

bin/varnishd/cache/cache_backend.c

@@ -233,15 +233,8 @@ vbe_dir_finish(VRT_CTX, VCL_BACKEND d)
 	CHECK_OBJ_NOTNULL(bo->htc, HTTP_CONN_MAGIC);
 	pfd = bo->htc->priv;
 	bo->htc->priv = NULL;
-	if (PFD_State(pfd) != PFD_STATE_USED) {
+	if (PFD_State(pfd) != PFD_STATE_USED)
 		AN(bo->htc->doclose);
-		if (bo->htc->doclose != SC_TX_PIPE) {
-#define SESS_CLOSE(U, l, err, desc)				\
-			if (bo->htc->doclose == SC_ ## U)	\
-				AN(err);
-#include "tbl/sess_close.h"
-		}
-	}
 	if (bo->htc->doclose != SC_NULL || bp->proxy_header != 0) {
 		VSLb(bo->vsl, SLT_BackendClose, "%d %s close", *PFD_Fd(pfd),
 		    VRT_BACKEND_string(d));