Fix #56: Remove redundant usedId from ERC721 implementation and update README

[nisedo]

The usedId mapping was declared but never properly utilized. Since the counter++ mechanism already ensures unique token IDs are minted, usedId was redundant and has been removed.

Changes:

• Remove usedId from IERC721Internal interface
• Remove usedId mapping from ERC721Compliant
• Remove usedId mapping from ERC721IncorrectMintable
• Remove usedId assertion from ERC721ExternalMintableProperties
• Update README to fix interface name (ITokenMock -> IERC721Internal) and remove usedId references from example code
• Format changed files with: npx prettier --write contracts/ERC721/util/IERC721Internal.sol contracts/ERC721/external/test/ERC721Compliant.sol contracts/ERC721/external/util/ERC721IncorrectMintable.sol contracts/ERC721/external/properties/ERC721ExternalMintableProperties.sol README.md

Fixes #56