Remove MLTool unique unit test.

arkindex/documents/tests/test_datasource.py

-from unittest.mock import MagicMock
-from django.test import TestCase
-from arkindex_common.ml_tool import MLTool, MLToolType
-from arkindex.documents.models import DataSource
-
-
-class TestDataSource(TestCase):
-
-    @classmethod
-    def setUpTestData(cls):
-        cls.ml_tool = MagicMock(spec=MLTool)
-        cls.ml_tool.type = MLToolType.Recognizer
-        cls.ml_tool.slug = 'muddy'
-        cls.ml_tool.name = 'Muddy Recognizer'
-        cls.ml_tool.version = '0.42'
-
-    def test_str(self):
-        ds = DataSource(
-            type=MLToolType.Classifier,
-            slug='something',
-            name='some classifier',
-            revision='1.2.3',
-            internal=False,
-        )
-        self.assertEqual(str(ds), 'something 1.2.3')