discover: Deprecate 'conf' user event

discover: Deprecate 'conf' user event

The 'conf' user event is functionally very similar to the 'url' event,
in that both events result in downloading a specified configuration file
and passing it to iterate_parsers().
The 'url' event additionally allows downloading files from a directory
path and is also accessed by the UI via pb-protocol, so remove the
'conf' event and associated functions in favour of 'url' and
device_handler_process_url().

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
(cherry picked from commit e2c92865481a5fb043c03c0cde625a96ca608699)