Update ERC20.sol (#2357)

Remove unused `Address` library import, references.

contracts/token/ERC20/ERC20.sol

@@ -5,7 +5,6 @@ pragma solidity ^0.6.0;
 import "../../GSN/Context.sol";
 import "./IERC20.sol";
 import "../../math/SafeMath.sol";
-import "../../utils/Address.sol";
 
 /**
  * @dev Implementation of the {IERC20} interface.
@@ -33,7 +32,6 @@ import "../../utils/Address.sol";
  */
 contract ERC20 is Context, IERC20 {
     using SafeMath for uint256;
-    using Address for address;
 
     mapping (address => uint256) private _balances;