[Cleanup] Remove RemoveSpawnGroup() from spawngroup.h (#3090)

# Notes
- This is unused.

zone/spawngroup.cpp

@@ -145,17 +145,6 @@ SpawnGroup *SpawnGroupList::GetSpawnGroup(uint32 in_id)
 	return (m_spawn_groups[in_id].get());
 }
 
-bool SpawnGroupList::RemoveSpawnGroup(uint32 in_id)
-{
-	if (m_spawn_groups.count(in_id) != 1) {
-		return (false);
-	}
-
-	m_spawn_groups.erase(in_id);
-
-	return (true);
-}
-
 void SpawnGroupList::ReloadSpawnGroups()
 {
 	ClearSpawnGroups();

zone/spawngroup.h

@@ -78,7 +78,6 @@ public:
 
 	void AddSpawnGroup(std::unique_ptr<SpawnGroup> &new_group);
 	SpawnGroup *GetSpawnGroup(uint32 id);
-	bool RemoveSpawnGroup(uint32 in_id);
 	void ClearSpawnGroups();
 	void ReloadSpawnGroups();
 private: