[Revert] Reverts Ninja (#15516)

* Revert "[Antag] add space ninja as midround antag (#14069)"

This reverts commit c1cda0dbf8.

* Revert "[Fix] move ninja objectives into NinjaRole (#15490)"

This reverts commit 251f429fb3.

Content.Client/Ninja/Systems/NinjaSystem.cs

@@ -1,12 +0,0 @@
-using Content.Shared.Ninja.Systems;
-
-namespace Content.Client.Ninja.Systems;
-
-/// <summary>
-/// Currently does nothing special clientside.
-/// All functionality is in shared and server.
-/// Only exists to prevent crashing.
-/// </summary>
-public sealed class NinjaSystem : SharedNinjaSystem
-{
-}