shell: No default realm discovery in Join Domain

On a server we don't need to deal with the default realm discovery
code. Lets just show the Join domain form right away.

Fixes #949
Closes #953
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
 * Adapted tests.
shell: Remove unused realms page HTML code

This is no longer used when the realms listing page was removed

See #862 and #911
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>