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/Dispel.cs

@@ -54,8 +54,6 @@ namespace Server.Spells.Sixth
                     Caster.SendLocalizedMessage(1010084); // The creature resisted the attempt to dispel it!
                 }
             }
-
-            FinishSequence();
         }
 
         public override void OnCast()

Projects/UOContent/Spells/Sixth/EnergyBolt.cs

@@ -59,8 +59,6 @@ namespace Server.Spells.Sixth
                 // Deal the damage
                 SpellHelper.Damage(this, m, damage, 0, 0, 0, 0, 100);
             }
-
-            FinishSequence();
         }
 
         public override void OnCast()

Projects/UOContent/Spells/Sixth/Explosion.cs

@@ -39,10 +39,8 @@ namespace Server.Spells.Sixth
                 SpellHelper.Turn(Caster, m);
                 SpellHelper.CheckReflect((int)Circle, Caster, ref m);
 
-                var t = new InternalTimer(this, Caster, defender, m).Start();
+                new InternalTimer(this, Caster, defender, m).Start();
             }
-
-            FinishSequence();
         }
 
         public override void OnCast()

Projects/UOContent/Spells/Sixth/Invisibility.cs

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

Projects/UOContent/Spells/Sixth/Mark.cs

@@ -55,8 +55,6 @@ namespace Server.Spells.Sixth
                 Caster.PlaySound(0x1FA);
                 Effects.SendLocationEffect(Caster, 14201, 16);
             }
-
-            FinishSequence();
         }
 
         public override void OnCast()

Projects/UOContent/Spells/Sixth/MassCurse.cs

@@ -54,8 +54,6 @@ namespace Server.Spells.Sixth
                     }
                 }
             }
-
-            FinishSequence();
         }
 
         public override void OnCast()

Projects/UOContent/Spells/Sixth/ParalyzeField.cs

@@ -67,8 +67,6 @@ public class ParalyzeFieldSpell : MagerySpell, ISpellTargetingPoint3D
                 );
             }
         }
-
-        FinishSequence();
     }
 
     public override void OnCast()

Projects/UOContent/Spells/Sixth/Reveal.cs

@@ -51,8 +51,6 @@ namespace Server.Spells.Sixth
                     }
                 }
             }
-
-            FinishSequence();
         }
 
         public override void OnCast()