[refactor] Remove credential response property "format"

[srosenda]

"format" is not a specified parameter of OpenID4VCI Credential Response: https://openid.net/specs/openid-4-verifiable-credential-issuance-1_0.html#name-credential-response.

Fixes #71.

Description of change

Remove unspecified optional property "format" from data structures that decode credential responses. The property value was always nil and it was not used for anything.

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• Project's automated test suite passes.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the readme
• My changes generate no new warnings
• I have added unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes