check: Remove url and external_identifier from mandatory fields

Related T1648

swh/deposit/api/private/deposit_check.py

@@ -124,8 +124,6 @@ class SWHChecksDeposit(SWHGetDepositAPI, SWHPrivateAPIView, DepositReadMixin):
 
         """
         required_fields = {
-            'url': False,
-            'external_identifier': False,
             'author': False,
         }
         alternate_fields = {

swh/deposit/tests/api/test_deposit_check.py

@@ -137,7 +137,7 @@ class CheckDepositTest(APITestCase, WithAuthTestCase,
         mandatory = details['metadata'][0]
         self.assertEqual(mandatory['summary'], MANDATORY_FIELDS_MISSING)
         self.assertEqual(set(mandatory['fields']),
-                         set(['url', 'external_identifier', 'author']))
+                         set(['author']))
         alternate = details['metadata'][1]
         self.assertEqual(alternate['summary'], ALTERNATE_FIELDS_MISSING)
         self.assertEqual(alternate['fields'], ['name or title'])