settings/example: remove 'example' settings plugin

settings/example: remove 'example' settings plugin

The 'example' settings plugin is (obviously) unused, but it is also
badly maintained and no longer best-practice in several ways:

  - it directly reads "NetworkManager.conf" instead of using NMConfig.
  - it parses device specs itself, instead of using
    nm_match_spec_split().
  - read_connections() doesn't ensure that loading a file
    does not replace a previously loaded one (due to conflicting
    UUID). In general the example doesn't show/handle the complexity
    of potential UUID conflicts.

Instead of fixing these issues it is better to ensure our main plugin
('keyfile') corresponds to current best practices. Should we ever add a
new pluginww, 'keyfile' should be the example.