Code Cleanup: Purge obsolete MapManager methods (#26279)

* GetGrid

* GridExists

* TryGetGrid

Content.Server/Anomaly/AnomalySystem.Generator.cs

@@ -24,7 +24,7 @@ namespace Content.Server.Anomaly;
 /// </summary>
 public sealed partial class AnomalySystem
 {
-    [Dependency] private readonly MapSystem _mapSystem = default!;
+    [Dependency] private readonly SharedMapSystem _mapSystem = default!;
     [Dependency] private readonly SharedTransformSystem _transform = default!;
 
     private void InitializeGenerator()