Code Cleanup: Purge obsolete MapManager methods (#26279)

* GetGrid

* GridExists

* TryGetGrid

Content.Server/NPC/Systems/NPCSteeringSystem.cs

@@ -47,7 +47,6 @@ public sealed partial class NPCSteeringSystem : SharedNPCSteeringSystem
     [Dependency] private readonly IAdminManager _admin = default!;
     [Dependency] private readonly IConfigurationManager _configManager = default!;
     [Dependency] private readonly IGameTiming _timing = default!;
-    [Dependency] private readonly IMapManager _mapManager = default!;
     [Dependency] private readonly IRobustRandom _random = default!;
     [Dependency] private readonly ClimbSystem _climb = default!;
     [Dependency] private readonly DoAfterSystem _doAfter = default!;