Stats: remove obsolete "simple" plugin special case

The proxy plugin has now implemented the "idle" event listener.

src/Stats.cxx

@@ -24,7 +24,6 @@
 #include "db/Selection.hxx"
 #include "db/DatabaseGlue.hxx"
 #include "db/DatabasePlugin.hxx"
-#include "db/DatabaseSimple.hxx"
 #include "util/Error.hxx"
 #include "system/Clock.hxx"
 #include "Log.hxx"
@@ -99,13 +98,6 @@ db_stats_print(Client &client)
 {
 	assert(GetDatabase() != nullptr);
 
-	if (!db_is_simple())
-		/* reload statistics if we're using the "proxy"
-		   database plugin */
-		/* TODO: move this into the "proxy" database plugin as
-		   an "idle" handler */
-		stats_invalidate();
-
 	if (!stats_update())
 		return;