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/Fifth/BladeSpirits.cs

@@ -37,8 +37,6 @@ namespace Server.Spells.Fifth
                 var duration = TimeSpan.FromSeconds(Core.AOS ? 120 : Utility.Random(80, 40));
                 BaseCreature.Summon(new BladeSpirits(), false, Caster, new Point3D(p), 0x212, duration);
             }
-
-            FinishSequence();
         }
 
         public override TimeSpan GetCastDelay()

Projects/UOContent/Spells/Fifth/DispelField.cs

@@ -47,8 +47,6 @@ namespace Server.Spells.Fifth
 
                 item.Delete();
             }
-
-            FinishSequence();
         }
 
         public override void OnCast()

Projects/UOContent/Spells/Fifth/MagicReflect.cs

@@ -99,8 +99,6 @@ namespace Server.Spells.Fifth
                         BuffInfo.AddBuff(Caster, new BuffInfo(BuffIcon.MagicReflection, 1075817, buffFormat, true));
                     }
                 }
-
-                FinishSequence();
             }
             else
             {
@@ -129,9 +127,9 @@ namespace Server.Spells.Fifth
                         Caster.SendLocalizedMessage(1005385); // The spell will not adhere to you at this time.
                     }
                 }
-
-                FinishSequence();
             }
+
+            FinishSequence();
         }
 
         public static void EndReflect(Mobile m)

Projects/UOContent/Spells/Fifth/MindBlast.cs

@@ -106,8 +106,6 @@ namespace Server.Spells.Fifth
 
                 SpellHelper.Damage(this, target, damage, 0, 0, 100, 0, 0);
             }
-
-            FinishSequence();
         }
 
         public override void OnCast()

Projects/UOContent/Spells/Fifth/Paralyze.cs

@@ -78,8 +78,6 @@ namespace Server.Spells.Fifth
 
                 HarmfulSpell(m);
             }
-
-            FinishSequence();
         }
 
         public override void OnCast()

Projects/UOContent/Spells/Fifth/PoisonField.cs

@@ -53,8 +53,6 @@ public class PoisonFieldSpell : MagerySpell, ISpellTargetingPoint3D
                 new PoisonField(itemID, targetLoc, Caster, Caster.Map, duration, i);
             }
         }
-
-        FinishSequence();
     }
 
     public override void OnCast()