Remove a whole bunch of non-sensical vlc_object_hold()

Remove a whole bunch of non-sensical vlc_object_hold()

In one case (noted by Erwan Tulou), this also fixes a large leak.

There is _absolutely_ no point in holding an object and releasing it
immediately. Holding an object only makes sense if:
 - the object cannot vanish while calling vlc_object_hold(), and
 - either:
   - the object could have otherwise vanished before
     vlc_object_release(), or
   - there is no way to avoid the (otherwise not needed)
     vlc_object_hold() because it comes from another component.

Conversely, it makes no sense to hold an object, if:
 - it could vanish already while calling hold (race condition)
   -> this is *invalid* and might crash,
 - there is already a valid reference to the object throughout.

I don't know in which case that code was, but the popup menu definitely
does invalid object access in some cases. For instance:
 - start playing a single video,
 - seek to a few seconds before the end,
 - open the popup menu, go to video / deinterlace,
 - wait for the video to finish and the playlist to stop,
 - click on "X" from the deinterlace submenu.
Oops.