music: Fix multidisc indexing

Kodi uses internally a right-shift operation to codify in a
single integer disc number and track number. This is taken
into account in this addon but, incorrectly, it is done for
discnumber > 1 only, while Kodi uses it for tracks in all
discs.

Fix this by directly codifying the track and disc number
for all discs.

This patch is based on investigation by user @neightwulf in
github.

jellyfin_kodi/objects/music.py

@@ -291,7 +291,6 @@ class Music(KodiDb):
         if obj['DatePlayed']:
             obj['DatePlayed'] = Local(obj['DatePlayed']).split('.')[0].replace('T', " ")
 
-        if obj['Disc'] != 1:
         obj['Index'] = obj['Disc'] * 2 ** 16 + obj['Index']
 
         if update: