Code Cleanup: Purge obsolete MapManager methods (#26279)

* GetGrid

* GridExists

* TryGetGrid

Content.Server/Decals/Commands/EditDecalCommand.cs

@@ -3,6 +3,7 @@ using Content.Server.Administration;
 using Content.Shared.Administration;
 using Robust.Shared.Console;
 using Robust.Shared.Map;
+using Robust.Shared.Map.Components;
 
 namespace Content.Server.Decals;
 
@@ -43,7 +44,7 @@ Possible modes are:\n
             return;
         }
 
-        if (!_mapManager.GridExists(gridId))
+        if (!_entManager.HasComponent<MapGridComponent>(gridId))
         {
             shell.WriteError($"No grid with gridId {gridId} exists.");
             return;