Remove SafeMath for uint256 (#555)

Co-authored-by: Kirill <pisarevkir@gmail.com>

Author: Neuti Yoo

contracts/ERC1155Safe.sol

@@ -1,7 +1,6 @@
 // SPDX-License-Identifier: LGPL-3.0-only
 pragma solidity 0.8.11;
 
-import "@openzeppelin/contracts/utils/math/SafeMath.sol";
 import "@openzeppelin/contracts/token/ERC1155/IERC1155.sol";
 import "@openzeppelin/contracts/token/ERC1155/extensions/ERC1155Burnable.sol";
 import "@openzeppelin/contracts/token/ERC1155/presets/ERC1155PresetMinterPauser.sol";
@@ -12,7 +11,6 @@ import "@openzeppelin/contracts/token/ERC1155/presets/ERC1155PresetMinterPauser.
     @notice This contract is intended to be used with ERC1155Handler contract.
  */
 contract ERC1155Safe {
-    using SafeMath for uint256;
 
     /**
         @notice Used to gain custoday of deposited token with batching.

contracts/ERC20Safe.sol

@@ -1,7 +1,6 @@
 // SPDX-License-Identifier: LGPL-3.0-only
 pragma solidity 0.8.11;
 
-import "@openzeppelin/contracts/utils/math/SafeMath.sol";
 import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 import "@openzeppelin/contracts/token/ERC20/presets/ERC20PresetMinterPauser.sol";
 import "@openzeppelin/contracts/token/ERC20/extensions/ERC20Burnable.sol";
@@ -12,7 +11,6 @@ import "@openzeppelin/contracts/token/ERC20/extensions/ERC20Burnable.sol";
     @notice This contract is intended to be used with ERC20Handler contract.
  */
 contract ERC20Safe {
-    using SafeMath for uint256;
 
     /**
         @notice Used to gain custody of deposited token.

contracts/ERC721Safe.sol

@@ -1,7 +1,6 @@
 // SPDX-License-Identifier: LGPL-3.0-only
 pragma solidity 0.8.11;
 
-import "@openzeppelin/contracts/utils/math/SafeMath.sol";
 import "@openzeppelin/contracts/token/ERC721/IERC721.sol";
 import "./ERC721MinterBurnerPauser.sol";
 
@@ -11,7 +10,6 @@ import "./ERC721MinterBurnerPauser.sol";
     @notice This contract is intended to be used with ERC721Handler contract.
  */
 contract ERC721Safe {
-    using SafeMath for uint256;
 
     /**
         @notice Used to gain custoday of deposited token.