[books] Remove unused max_length parameter

vrobbler/apps/books/models.py

@@ -153,8 +153,8 @@ class Book(LongPlayScrobblableMixin):
     comicvine_id = models.CharField(max_length=255, **BNULL)
     readcomics_url = models.CharField(max_length=255, **BNULL)
     next_readcomics_url = models.CharField(max_length=255, **BNULL)
-    issue_number = models.IntegerField(max_length=5, **BNULL)
-    volume_number = models.IntegerField(max_length=5, **BNULL)
+    issue_number = models.IntegerField(**BNULL)
+    volume_number = models.IntegerField(**BNULL)
     # OpenLibrary
     openlibrary_id = models.CharField(max_length=255, **BNULL)
     cover = models.ImageField(upload_to="books/covers/", **BNULL)