Don't use primary key for scrobbled tracks embeds

lib/last_fm/album.ex

@@ -7,6 +7,7 @@ defmodule LastFm.Album do
           musicbrainz_id: String.t(),
           title: String.t()
         }
+  @primary_key false
   embedded_schema do
     field :musicbrainz_id, :string
     field :title, :string

lib/last_fm/artist.ex

@@ -15,6 +15,7 @@ defmodule LastFm.Artist do
           image_data_hash: String.t() | nil
         }
 
+  @primary_key false
   embedded_schema do
     field :musicbrainz_id, :string
     field :name, :string