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/Sixth/Invisibility.cs

@@ -67,8 +67,6 @@ namespace Server.Spells.Sixth
 
                 _table[m] = timerToken;
             }
-
-            FinishSequence();
         }
 
         public override bool CheckCast()