dont assert on a failing rmdir

We may not have permission to unlink vp.dir

$ /etc/init.d/varnish_master restart
Stopping Varnish Cache:                                    [FAILED]
Starting Varnish Cache: Assert error in mgt_VccCompile(), mgt/mgt_vcc.c line 302:
  Condition((rmdir(vp.dir)) == 0) not true.
  errno = 13 (Permission denied)

bin/varnishd/mgt/mgt_vcc.c

@@ -299,7 +299,7 @@ mgt_VccCompile(struct cli *cli, const char *vclname, const char *vclsrc,
 	if (status || C_flag) {
 		(void)unlink(vp.libfile);
 		free(vp.libfile);
-		AZ(rmdir(vp.dir));
+		(void)rmdir(vp.dir);
 		free(vp.dir);
 		if (status) {
 			VCLI_Out(cli, "VCL compilation failed");