Remove deprecated required abstract factory class.

This functionality is being replaced with methods in the
`validate` class.

datatest/__init__.py

@@ -9,7 +9,6 @@
 from .difference import *  # Difference classes.
 from .allowance import *   # Allowance context mangers.
 from ._required import group_requirement
-from ._required import required
 from ._predicate import Predicate
 
 # Unittest-style API

datatest/_required.py

@@ -887,38 +887,3 @@ def check_group(self, group):
         differences = (Extra(element) for element in extras)
         description = 'may contain only elements of given superset'
         return differences, description
-
-
-class required(abc.ABC):
-    """:class:`required` is an abstract factory class that returns
-    requirement objects. It contains several factory methods that can
-    be called directly to instantiate specific requirement types.
-    """
-    def __new__(cls, *args, **kwds):
-        msg = ("Can't instantiate abstract class required, use factory "
-               "methods like required.unique(), required.subset(), etc.")
-        raise TypeError(msg)
-
-    @classmethod
-    def unique(cls):
-        """Requires that elements are unique.
-
-        .. code-block:: python
-            :emphasize-lines: 6
-
-            from datatest import validate
-            from datatest import required
-
-            data = [1, 2, 3, ...]
-
-            validate(data, required.unique())
-        """
-        return RequiredUnique()
-
-    @classmethod
-    def subset(cls, set):
-        pass
-
-    @classmethod
-    def superset(cls, set):
-        pass

tests/test_required.py

@@ -31,7 +31,6 @@
 from datatest._required import RequiredUnique
 from datatest._required import RequiredSubset
 from datatest._required import RequiredSuperset
-from datatest._required import required
 from datatest.difference import NOTFOUND
 
 
@@ -1669,22 +1668,3 @@ def test_single_element_handling(self):
         diff, desc = requirement((3, 4))  # <- Tuple is single element.
         diff = sorted(diff, key=lambda x: x.args)
         self.assertEqual(diff, [Extra((3, 4))])
-
-
-class TestRequiredFactory(unittest.TestCase):
-    def test_bad_init(self):
-        """The `required` factory should not be instantiated directly."""
-        with self.assertRaises(TypeError):
-            required('foo')
-
-    def test_unique(self):
-        requirement = required.unique()
-        self.assertIsInstance(requirement, RequiredUnique)
-
-    @unittest.skip('not yet implemented')
-    def test_subset(self):
-        pass
-
-    @unittest.skip('not yet implemented')
-    def test_superset(self):
-        pass