fix: Fixes double calls with Target cancel and spell sequences (#1840)

### Summary
- Cleans up target cancellation being called incorrectly.
- An invalid target type (which should never happen), now calls `OnTargetUntargetable` instead of `OnTargetCanceled`
- Removes double calls to `FinishSequence` in Spells.

Projects/UOContent/Spells/Necromancy/MindRot.cs

@@ -59,8 +59,6 @@ public class MindRotSpell : NecromancerSpell, ISpellTargetingMobile
 
             HarmfulSpell(m);
         }
-
-        FinishSequence();
     }
 
     public override void OnCast()
@@ -133,4 +131,4 @@ public class MRExpireTimer : Timer
             Stop();
         }
     }
-}
+}