Fixed IsUniqueMonsterValid() so that it doesn't wrongly return false for Zhar the Mad and the Warlord of Blood (#8104)

Source/monsters/validation.cpp

@@ -55,10 +55,6 @@ bool IsUniqueMonsterValid(const Monster &monster)
 		return false;
 	}
 
-	if (uniqueMonsterData.mlevel != 0 && uniqueMonsterData.mlevel != currlevel) {
-		return false;
-	}
-
 	return true;
 }