Normalize return variable names in IERC721Enumerable (#3079)

* Name missing return variable in IERC721Enumerable

For consistency among all functions.

* remove return variable names

contracts/token/ERC721/extensions/IERC721Enumerable.sol

@@ -19,7 +19,7 @@ interface IERC721Enumerable is IERC721 {
      * @dev Returns a token ID owned by `owner` at a given `index` of its token list.
      * Use along with {balanceOf} to enumerate all of ``owner``'s tokens.
      */
-    function tokenOfOwnerByIndex(address owner, uint256 index) external view returns (uint256 tokenId);
+    function tokenOfOwnerByIndex(address owner, uint256 index) external view returns (uint256);
 
     /**
      * @dev Returns a token ID at a given `index` of all the tokens stored by the contract.