Don't pre-create github provider on registration

[eleftherias]

Note: This is not about removing or changing the legacy OAuth GitHub provider.

When a user registers for Minder, currently a provider gets pre-created for them.
After running minder auth login for the first time, if they run minder provider list they will see the following:

+--------+--------------------------------------+---------+--------------------------------+
|  NAME  |               PROJECT                | VERSION |           IMPLEMENTS           |
+--------+--------------------------------------+---------+--------------------------------+
| github | 9f2ebd65-0e49-4ed1-968f-f661b9987261 | v1      | github, git, rest, repo-lister |
+--------+--------------------------------------+---------+--------------------------------+

This is confusing, because the user has not enrolled any providers, yet they see one already exists.
Furthermore, this provider cannot be used, since it does not have a valid credential associated with it. It is left over from an earlier version of Minder where the provider needed to pre-exist before it could be enrolled.

We should remove that built-in provider, so that when a user registers for the first time, they do not have any providers pre-created.

Enrolling the github provider should continue to work as before.

[eleftherias]

Closed via #4367