Core/Battleground: Fix crash added in c28345e279f31af2fa32df2411325ccfabe06fc8

Fix a double-delete crash added in c28345e279f31af2fa32df2411325ccfabe06fc8 caused by copy-constructor copying pointers across different Battleground instances.

Valgrind log:
 Invalid read of size 8
  at : Battleground::~Battleground() (Battleground.cpp:231)
  by : BattlegroundWS::~BattlegroundWS() (BattlegroundWS.cpp:74)
  by : BattlegroundWS::~BattlegroundWS() (BattlegroundWS.cpp:74)
  by : BattlegroundMgr::Update(unsigned int) (BattlegroundMgr.cpp:103)
  by : World::Update(unsigned int) (World.cpp:2067)
 Address 0x5675d140 is 0 bytes inside a block of size 24 free'd
  at : operator delete(void*) (vg_replace_malloc.c:509)
  by : ArenaTeamScore::~ArenaTeamScore() (ArenaScore.h:64)
  by : Battleground::~Battleground() (Battleground.cpp:231)
  by : BattlegroundWS::~BattlegroundWS() (BattlegroundWS.cpp:74)
  by : BattlegroundWS::~BattlegroundWS() (BattlegroundWS.cpp:74)
  by : BattlegroundMgr::Update(unsigned int) (BattlegroundMgr.cpp:103)