Core/Spells: Removed MECHANIC_IMMUNE_SHIELD + MECHANIC_BANISH special case check in Spell::CheckCasterAuras and named possibly related spell attribute

Port From (https://github.com/TrinityCore/TrinityCore/commit/721103c1c9649fc769a4230216b871319a5f7643)

Source/Framework/Constants/Spells/SpellConst.cs

@@ -1977,7 +1977,7 @@ namespace Framework.Constants
         CooldownIgnoresRangedWeapon = 0x100, // Cooldown Ignores Ranged Weapon
         NotInArena = 0x200, // 9 Not In Arena
         TargetMustBeGrounded = 0x400, // Target Must Be Grounded
-        Unk11 = 0x800, // 11
+        AllowWhileBanishedAuraState = 0x800, // Doesn't seem to be doing anything, banish behaves like a regular stun now - tested on patch 10.2.7 with spell 17767 (doesn't have this attribute, only SPELL_ATTR5_ALLOW_WHILE_STUNNED and was castable while banished)
         Unk12 = 0x1000, // 12
         Slam = 0x2000, // 13
         UsableInRatedBattlegrounds = 0x4000, // 14 Can Be Used In Rated Battlegrounds

Source/Game/Spells/Spell.cs

@@ -6309,8 +6309,6 @@ namespace Game.Spells
                 }
                 else if (!CheckSpellCancelsStun(ref param1))
                     result = SpellCastResult.Stunned;
-                else if ((m_spellInfo.Mechanic & Mechanics.ImmuneShield) != 0 && m_caster.IsUnit() && m_caster.ToUnit().HasAuraWithMechanic(1 << (int)Mechanics.Banish))
-                    result = SpellCastResult.Stunned;
             }
             else if (unitCaster.IsSilenced(m_spellSchoolMask) && (m_spellInfo.PreventionType & SpellPreventionType.Silence) != 0 && !CheckSpellCancelsSilence(ref param1))
                 result = SpellCastResult.Silenced;