Simplify inheritance to avoid overrides

(cherry picked from commit fc01c51c130b9f9a8638cb3e86a43175bdf21c36)

docs/modules/ROOT/pages/erc721.adoc

@@ -20,7 +20,7 @@ import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
 import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";
 import "@openzeppelin/contracts/utils/Counters.sol";
 
-contract GameItem is ERC721, ERC721URIStorage {
+contract GameItem is ERC721URIStorage {
     using Counters for Counters.Counter;
     Counters.Counter private _tokenIds;