Delete credential if effective date == expiry date

[ianco]

When aries-vcr receives a new credential, it checks for an existing credential and updates the expiry date on the previous credential.

If the effective and expiry dates are the same on the credential, delete it (rather than storing a "zero-length" credential).

This is to support processing of data fixes in the BC Reg source data - if the BC Reg DBA's do a manual data fix, we can simply re-process the company, and any credentials in orgbook that are getting replaced will be automatically deleted.