_calculateTotalEarningPower formula can be simplified

[CergyK]

Description

_calculateTotalEarningPower can be simplified to the equivalent formula:

GovernanceStaker.sol#L759-L768:

  function _calculateTotalEarningPower(
    uint256 _depositOldEarningPower,
    uint256 _depositNewEarningPower,
    uint256 _totalEarningPower
  ) internal pure returns (uint256 _newTotalEarningPower) {
-    if (_depositNewEarningPower >= _depositOldEarningPower) {
-      return _totalEarningPower + (_depositNewEarningPower - _depositOldEarningPower);
-    }
-    return _totalEarningPower - (_depositOldEarningPower - _depositNewEarningPower);
+    return _totalEarningPower + _depositNewEarningPower - _depositOldEarningPower;
  }

[alexkeating]

Will fix

[CergyK]

Fixed by withtally/staker#89