simplify code to reduce deploy gas cost

contracts/intf/IERC20.sol

@@ -70,18 +70,4 @@ interface IERC20 {
         address recipient,
         uint256 amount
     ) external returns (bool);
-
-    /**
-     * @dev Emitted when `value` tokens are moved from one account (`from`) to
-     * another (`to`).
-     *
-     * Note that `value` may be zero.
-     */
-    event Transfer(address indexed from, address indexed to, uint256 value);
-
-    /**
-     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
-     * a call to {approve}. `value` is the new allowance.
-     */
-    event Approval(address indexed owner, address indexed spender, uint256 value);
 }