phabricator: Remove custom json unmarshalling

Created by: ryanslade

The custom unmarshaller on the Repo type was removed. Instead, we marshal into an apiRepo struct and convert that into a Repo struct. Having a custom unmarshal on the Repo type meant that when it was marshalled into our database it couldn't be unmarshalled again. This in turn broke our ability to build a clone url from a stored Phabricator repo.

Spotted while investigating https://github.com/sourcegraph/customer/issues/371 but unlikely to be the fix.