Core/BG: use new GetUniqueBracketID() method to store statistics

This is because now BattleGrounds with different types and same level range has different m_BracketId

NOTE: GetUniqueBracketID() method should be rewritten for 4.3.4. I'll do that once this PR is merged in master and once master will be merged in 4.3.4.
change to `uint8 GetUniqueBracketId() const { return GetMinLevel() / 10; }` and remove the .cpp definition