Core/Battleground: fix arena points distribution for non-80 Call To Arms...

Notes:

- defining BG_AWARD_ARENA_POINTS_MIN_LEVEL as 71 had no sense, because the only function which uses that value is:

```
bool CanAwardArenaPoints() const { return m_LevelMin >= BG_AWARD_ARENA_POINTS_MIN_LEVEL; }
```

and it will always returns FALSE using 71, except for level 80 BGs, because m_LevelMin are always 10, 20, 30, 40, 50, 60, 70 or 80.

- the client seems to confirm that all level brackets should be rewarded with arena points, even at 10-19: 

![alt text](http://s30.postimg.org/fnf2e4z75/twink10.png "10-19 join BG client window")