Drop domain key on PhameBlog

Summary: Right now you can't create two blogs without a domain name, since it has a unique key on the column. Removing the key.

Test Plan: Create two blogs with no domain name, works as expected. Create two blogs with cat.dog as domain name, get duplicate domain error.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9360

Differential Revision: https://secure.phabricator.com/D14915