chore: remove reference to deprecated HierarchyMixin

This has been a no-op for a long time anyway, since HierarchyMixin
has been explicitly mixed into XBlock for as long as I remember.

Ref: openedx/XBlock#714

xmodule/tests/xml/factories.py

@@ -9,7 +9,6 @@
 from factory import Factory, Sequence, lazy_attribute, post_generation
 from fs.osfs import OSFS
 from lxml import etree
-from xblock.mixins import HierarchyMixin
 
 from xmodule.modulestore.inheritance import InheritanceMixin
 from xmodule.x_module import XModuleMixin
@@ -70,7 +69,7 @@ class Meta:
         model = XmlImportData
 
     filesystem = OSFS(mkdtemp())
-    xblock_mixins = (InheritanceMixin, XModuleMixin, HierarchyMixin)
+    xblock_mixins = (InheritanceMixin, XModuleMixin)
     url_name = Sequence(str)
     attribs = {}
     policy = {}