Remove notion of "Default organization" from `h` codebase

Depends on https://github.com/hypothesis/h/pull/5223 and https://github.com/hypothesis/h/pull/5228

After we make `organization` an _optional_ relationship on a group and migrate all private groups to not have an organization, we should come back and clean up the codebase's notion of a "default" organization.

To wit:

* [ ] `Organization.default` is probably not a necessary method (`h.models.organization`)
* [ ] There may be other instances of "default" organization in the codebase—grepping time!
* [ ] Possibly remove the "default" Hypothesis organization from the DB?

Read PRs 5223 and 5228 first to understand the organization relationship and private-group migration. Then inspect h.models.organization and grep the h codebase for other references to a default organization. Done means removing unnecessary default-organization behavior and deciding whether the default Hypothesis organization should be removed from the database.

Written by the indexing model from the issue text.