Remove code in `lib/requests.js` that handles `verifiableCredential`

[aljones15]

The VC-API now requires that 201 issuer responses contains the property verifiableCredential so the code in lib/requests.js will need to remove the exception it made previously.

[BigBlueHat]

https://github.com/w3c-ccg/vc-test-suite-implementations/blob/main/lib/requests.js#L114-L118