Fix scrobble tracks schema

Remove the primary key, and just set a unique index on scrobbled time
and title, which allows skipping duplicates.

lib/last_fm/import.ex

@@ -20,7 +20,8 @@ defmodule LastFm.Import do
       # HACK: if two tracks happen to have the exact same scrobbled_at_uts,
       # we move it by a sec.
       MusicLibrary.Repo.insert_all(LastFm.Track, track_params,
-        on_conflict: [inc: [scrobbled_at_uts: -1]]
+        on_conflict: :nothing,
+        conflict_target: [:scrobbled_at_uts, :title]
       )
     end
   end