[identity] don't check reserved_usernames.json when registering existing users from keyserver
If a username is already registered with ashoat's keyserver, there's no need to check if it's in reserved_usernames.json. We should just allow the username to be registered on the identity service via the reserved registration workflow.
Test Plan:
- Registered username book with my keyserver by setting usingCSAT to false and modifying the reserved usernames js file. Keyserver added username to reserved usernames table on staging identity.
- Flipped usingCSAT to true, was registered with staging identity
Reviewers: ashoat, will, bartek
Reviewed By: will
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D13276