Remove BuiltinAssertionError

We used to have this when we where patching the real Python
AssertionError for use with reinterpret, but reinterpret is now
gone so we no longer need this as it is not used by rewrite.

testing/python/raises.py

@@ -26,14 +26,6 @@ class TestRaises:
         except pytest.raises.Exception:
             pass
 
-    def test_raises_flip_builtin_AssertionError(self):
-        # we replace AssertionError on python level
-        # however c code might still raise the builtin one
-        from _pytest.assertion.util import BuiltinAssertionError # noqa
-        pytest.raises(AssertionError,"""
-            raise BuiltinAssertionError
-        """)
-
     def test_raises_as_contextmanager(self, testdir):
         testdir.makepyfile("""
             from __future__ import with_statement