cleaned up unsued variable definition (#753)

contracts/token/ERC20/BasicToken.sol

@@ -43,7 +43,7 @@ contract BasicToken is ERC20Basic {
   * @param _owner The address to query the the balance of.
   * @return An uint256 representing the amount owned by the passed address.
   */
-  function balanceOf(address _owner) public view returns (uint256 balance) {
+  function balanceOf(address _owner) public view returns (uint256) {
     return balances[_owner];
   }