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/Strangle.cs

@@ -107,8 +107,6 @@ public class StrangleSpell : NecromancerSpell, ISpellTargetingMobile
 
         var t_Duration = TimeSpan.FromSeconds(length);
         BuffInfo.AddBuff(m, new BuffInfo(BuffIcon.Strangle, 1075794, 1075795, t_Duration, m, args));
-
-        FinishSequence();
     }
 
     public override void OnCast()
@@ -226,4 +224,4 @@ public class StrangleSpell : NecromancerSpell, ISpellTargetingMobile
             }
         }
     }
-}
+}