lib: remove unused public `enum bt_plugin_status`

lib: remove unused public `enum bt_plugin_status`

Also:

* Make the plugin's user exit function return nothing. It's not like we
  can cancel this anyway.

* Make the plugin's user init function return
  `enum bt_plugin_init_status`, an enumeration reserved for this
  function, in case `enum bt_plugin_status` becomes public in the
  future.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>