partly reverted old 3f9adcb: don't mix disruption and concentration

once we're in that branch, we already know the checks failed, so
don't give them another chance with a different calculation

gemrb/core/GameScript/GSUtils.cpp

@@ -2575,31 +2575,13 @@ inline static bool InterruptSpellcasting(Scriptable* Sender) {
 
 	// ouch, we got hit
 	if (Sender->InterruptCasting) {
-		int roll = 0;
-
-		// iwd2 does an extra concentration check first:
-		// d20 + Concentration Skill Level + Constitution bonus (+4 Combat casting feat) >= 15 + spell level
-		if (core->HasFeature(GF_3ED_RULES)) {
-			roll = core->Roll(1, 20, 0); // TODO: check if the original does a lucky roll
-			roll += caster->GetStat(IE_CONCENTRATION);
-			roll += caster->GetAbilityBonus(IE_CON);
-			if (caster->HasFeat(FEAT_COMBAT_CASTING)) {
-				roll += 4;
-			}
-			Spell* spl = gamedata->GetSpell(Sender->SpellResRef, true);
-			if (!spl) return false;
-			roll -= spl->SpellLevel;
-			gamedata->FreeSpell(spl, Sender->SpellResRef, false);
-		}
-		if (roll < 15) {
-			if (caster->InParty) {
-				displaymsg->DisplayConstantString(STR_SPELLDISRUPT, DMC_WHITE, Sender);
-			} else {
-				displaymsg->DisplayConstantStringName(STR_SPELL_FAILED, DMC_WHITE, Sender);
-			}
-			DisplayStringCore(Sender, VB_SPELL_DISRUPTED, DS_CONSOLE|DS_CONST );
-			return true;
+		if (caster->InParty) {
+			displaymsg->DisplayConstantString(STR_SPELLDISRUPT, DMC_WHITE, Sender);
+		} else {
+			displaymsg->DisplayConstantStringName(STR_SPELL_FAILED, DMC_WHITE, Sender);
 		}
+		DisplayStringCore(Sender, VB_SPELL_DISRUPTED, DS_CONSOLE|DS_CONST );
+		return true;
 	}
 
 	// abort casting on invisible or dead targets