Core/Misc: Revert a static analysis "fix"

Revert assertion check added in 5dfd07fa4f , the issue reported by Coverity is a false positive.

src/server/game/Spells/Auras/SpellAuras.cpp

@@ -332,9 +332,6 @@ Aura* Aura::Create(SpellInfo const* spellproto, uint32 effMask, WorldObject* own
     else
         casterGUID = caster->GetGUID();
 
-    // at this point of Aura::Create() there MUST be a valid caster
-    ASSERT(caster);
-
     // check if aura can be owned by owner
     if (owner->isType(TYPEMASK_UNIT))
         if (!owner->IsInWorld() || ((Unit*)owner)->IsDuringRemoveFromWorld())