OPTIM: stats: avoid the calculation of a useless link on tracking servers in maintenance

Commit f465994198 removed the "via" link when a tracking server is in maintenance, but
still calculated an empty link that no one can use. We can safely remove it.
diff --git a/src/dumpstats.c b/src/dumpstats.c
index b26ec25..c76e115 100644
--- a/src/dumpstats.c
+++ b/src/dumpstats.c
@@ -2983,9 +2983,7 @@
 		}
 		else if (sv != ref) {
 			if (sv->state & SRV_MAINTAIN)
-				chunk_appendf(&trash,
-					      "<td class=ac colspan=3><a class=lfsb href=\"#%s/%s\"></a></td>",
-					      ref->proxy->id, ref->id);
+				chunk_appendf(&trash, "<td class=ac colspan=3></td>");
 			else
 				chunk_appendf(&trash,
 					      "<td class=ac colspan=3><a class=lfsb href=\"#%s/%s\">via %s/%s</a></td>",