Comment out of owner and spender [key] attributes in erc20 contract

[mikiw]

Usage related changes

• in erc20 contract in struct Approval, owner and spender [key] attributes were removed similar to
  https://github.com/starknet-io/starkgate-contracts/blob/d62a255307d2f3de65665f18316766a2c69ead78/src/openzeppelin/token/erc20_v070/erc20.cairo#L100
• erc20 class hash was hardcoded to be the same as OZ class hash which is https://docs.openzeppelin.com/contracts-cairo/0.8.1/presets although it should be different due to commented key attributes and add of mintable feature

Checklist:

• Applied formatting - ./scripts/format.sh
• No linter errors - ./scripts/clippy_check.sh
• No unused dependencies - ./scripts/check_unused_deps.sh
• Performed code self-review
• Rebased to the last commit of the target branch (or merged it into my branch)
• Documented the changes
• Linked the issues which this PR resolves
• Checked the TODO section in README.md if this PR resolves it
• Updated the tests
• All tests are passing - cargo test