Removed unneeded nullptr check

(cherry picked from commit 20215a8c421fd93d00233213966399a0c76e626a)

src/server/scripts/EasternKingdoms/Stratholme/boss_baron_rivendare.cpp

@@ -106,7 +106,7 @@ public:
         void AttackStart(Unit* who) override
         {
             //can't use entercombat(), boss' dmg aura sets near players in combat, before entering the room's door
-            if (instance && instance->GetData(TYPE_BARON) == NOT_STARTED)
+            if (instance->GetData(TYPE_BARON) == NOT_STARTED)
                 instance->SetData(TYPE_BARON, IN_PROGRESS);
             ScriptedAI::AttackStart(who);
         }