Do not select active VCL automatically in the child process

Do not select the first VCL present as the active VCL in the child
process. Instead it should always use what the mgt process tells it
through the explicit 'vcl.use' commands it will send.

Previously, the child could select a cold VCL as the active VCL if that
happened to be the first one present to it during restarts. This could
lead to asserts as that is not an allowed state.

bin/varnishd/cache/cache_vcl.c

@@ -707,10 +707,6 @@ vcl_load(struct cli *cli,
 
 	VCLI_Out(cli, "Loaded \"%s\" as \"%s\"", fn , name);
 	VTAILQ_INSERT_TAIL(&vcl_head, vcl, list);
-	Lck_Lock(&vcl_mtx);
-	if (vcl_active == NULL)
-		vcl_active = vcl;
-	Lck_Unlock(&vcl_mtx);
 	VSC_C_main->n_vcl++;
 	VSC_C_main->n_vcl_avail++;
 }