Remove unused variable from the Book sync task.

lib/decentralised_book_index/sync/sync_tasks/sync_books_task.ex

@@ -46,7 +46,7 @@ defmodule DecentralisedBookIndex.SyncTasks.SyncBooksTask do
   end
 
   defp get_author_roles_url(json) do
-    url = get_in(json, ["relationships", "author_roles", "links", "related"])
+    get_in(json, ["relationships", "author_roles", "links", "related"])
   end
 
   defp get_author_roles(json) do
@@ -64,7 +64,7 @@ defmodule DecentralisedBookIndex.SyncTasks.SyncBooksTask do
   end
 
   defp get_bids_url(json) do
-    url = get_in(json, ["relationships", "bids", "links", "related"])
+    get_in(json, ["relationships", "bids", "links", "related"])
   end
 
   defp get_bids(json) do