Fix code comments (#2658)

contracts/utils/math/Math.sol

@@ -25,7 +25,7 @@ library Math {
      * zero.
      */
     function average(uint256 a, uint256 b) internal pure returns (uint256) {
-        // (a + b) / 2 can overflow, so we distribute
+        // (a + b) / 2 can overflow, so we distribute.
         return (a / 2) + (b / 2) + ((a % 2 + b % 2) / 2);
     }
 }

contracts/utils/math/SafeMath.sol

@@ -175,10 +175,6 @@ library SafeMath {
      * @dev Returns the integer division of two unsigned integers, reverting with custom message on
      * division by zero. The result is rounded towards zero.
      *
-     * Counterpart to Solidity's `%` operator. This function uses a `revert`
-     * opcode (which leaves remaining gas untouched) while Solidity uses an
-     * invalid opcode to revert (consuming all remaining gas).
-     *
      * Counterpart to Solidity's `/` operator. Note: this function uses a
      * `revert` opcode (which leaves remaining gas untouched) while Solidity
      * uses an invalid opcode to revert (consuming all remaining gas).