Docs: Remove wrong / confusing auth docs.
Closes #2028
See merge request pleroma/pleroma!2860
### :auth
-* `Pleroma.Web.Auth.PleromaAuthenticator`: default database authenticator.
-* `Pleroma.Web.Auth.LDAPAuthenticator`: LDAP authentication.
-
Authentication / authorization settings.
* `auth_template`: authentication form template. By default it's `show.html` which corresponds to `lib/pleroma/web/templates/o_auth/o_auth/show.html.eex`.