removes unused dependency (#2603)

contracts/token/ERC721/ERC721.sol

@@ -5,7 +5,6 @@ pragma solidity ^0.8.0;
 import "./IERC721.sol";
 import "./IERC721Receiver.sol";
 import "./extensions/IERC721Metadata.sol";
-import "./extensions/IERC721Enumerable.sol";
 import "../../utils/Address.sol";
 import "../../utils/Context.sol";
 import "../../utils/Strings.sol";