Remove mox in favour of built-in Req stubs

lib/music_brainz.ex

@@ -1,9 +1,11 @@
 defmodule MusicBrainz do
+  alias MusicBrainz.API
+
   def search_release_group(query, opts \\ []) do
     limit = Keyword.get(opts, :limit, 20)
     offset = Keyword.get(opts, :offset, 0)
 
-    music_brainz_config().api.search_release_group(
+    API.search_release_group(
       query,
       [limit: limit, offset: offset],
       music_brainz_config()
@@ -11,23 +13,23 @@ defmodule MusicBrainz do
   end
 
   def get_release_group(musicbrainz_id) do
-    music_brainz_config().api.get_release_group(musicbrainz_id, music_brainz_config())
+    API.get_release_group(musicbrainz_id, music_brainz_config())
   end
 
   def get_releases(musicbrainz_id, opts) do
-    music_brainz_config().api.get_releases(musicbrainz_id, opts, music_brainz_config())
+    API.get_releases(musicbrainz_id, opts, music_brainz_config())
   end
 
   def get_release(musicbrainz_id) do
-    music_brainz_config().api.get_release(musicbrainz_id, music_brainz_config())
+    API.get_release(musicbrainz_id, music_brainz_config())
   end
 
   def search_release_by_barcode(barcode) do
-    music_brainz_config().api.search_release_by_barcode(barcode, music_brainz_config())
+    API.search_release_by_barcode(barcode, music_brainz_config())
   end
 
   def get_cover_art(id_or_url) do
-    music_brainz_config().api.get_cover_art(id_or_url, music_brainz_config())
+    API.get_cover_art(id_or_url, music_brainz_config())
   end
 
   defp music_brainz_config, do: MusicBrainz.Config.resolve(:music_library)