[8381] Some refactoring work in Unit::m_currentSpells code.

* Restrict access, use enum args, move some repeated code parts to function.
* Make m_selfContainer set only part of Unit::SetCurrentCastedSpell

src/game/Spell.cpp

@@ -2265,7 +2265,6 @@ void Spell::prepare(SpellCastTargets const* targets, Aura* triggeredByAura)
     else
     {
         m_caster->SetCurrentCastedSpell( this );
-        m_selfContainer = &(m_caster->m_currentSpells[GetCurrentContainer()]);
         SendSpellStart();
     }
 }