Refactor: Get rid of pytrakt_extensions (port functionality to pytrakt)

[Elan Ruusamäe]

Feature description

Use case

Similarly to #1826

There's still some code left that's hacking pytrakt internals:

• https://github.com/Taxel/PlexTraktSync/blob/dd1bfa22c52bc8eb651a7159bf43f9ef7226c33b/plextraktsync/pytrakt_extensions.py

More specifically:

• watched_shows:

  PlexTraktSync/plextraktsync/trakt/TraktApi.py

  Lines 120 to 121 in dd1bfa2

   def watched_shows(self): 
       return pytrakt_extensions.allwatched() 

• collected_shows:

  PlexTraktSync/plextraktsync/trakt/TraktApi.py

  Lines 126 to 127 in dd1bfa2

   def collected_shows(self): 
       return pytrakt_extensions.allcollected() 

The plan is to get rid of it and add missing functionality to pytrakt if needed.