Remove deprecated CarpetExtension.customSettingsManager

All extensions I've found using a custom manager have migrated

src/main/java/carpet/CarpetServer.java

@@ -226,14 +226,5 @@ public class CarpetServer // static for now - easier to handle all around the co
         scriptServer.reload(server);
         extensions.forEach(e -> e.onReload(server));
     }
-    
-    private static final Set<CarpetExtension> warnedOutdatedManagerProviders = new HashSet<>();
-    static void warnOutdatedManager(CarpetExtension ext)
-    {
-        if (warnedOutdatedManagerProviders.add(ext))
-            CarpetSettings.LOG.warn("""
-                    %s is providing a SettingsManager from an outdated method in CarpetExtension!
-                    This behaviour will not work in later Carpet versions and the manager won't be registered!""".formatted(ext.getClass().getName()));
-    }
 }