Core/Unit: 229444b74a follow-up

- IsFocusing is made virtual again, so there's no need to keep a duplicated function

This reverts commit 5043639c56.

src/server/game/Entities/Unit/Unit.cpp

@@ -3325,6 +3325,9 @@ bool Unit::IsMovementPreventedByCasting() const
             if (spell->GetSpellInfo()->IsMoveAllowedChannel())
                 return false;
 
+    if (const_cast<Unit*>(this)->IsFocusing(nullptr, true))
+        return false;
+
     // prohibit movement for all other spell casts
     return true;
 }