Code Cleanup: Purge obsolete MapManager methods (#26279)

* GetGrid

* GridExists

* TryGetGrid

Content.Server/Decals/Commands/RemoveDecalCommand.cs

@@ -2,6 +2,7 @@ using Content.Server.Administration;
 using Content.Shared.Administration;
 using Robust.Shared.Console;
 using Robust.Shared.Map;
+using Robust.Shared.Map.Components;
 using SQLitePCL;
 
 namespace Content.Server.Decals.Commands
@@ -31,7 +32,7 @@ namespace Content.Server.Decals.Commands
 
             if (!NetEntity.TryParse(args[1], out var rawGridIdNet) ||
                 !_entManager.TryGetEntity(rawGridIdNet, out var rawGridId) ||
-                !_mapManager.GridExists(rawGridId))
+                !_entManager.HasComponent<MapGridComponent>(rawGridId))
             {
                 shell.WriteError("Failed parsing gridId.");
                 return;