test: remove only_xmodules

This function was only used by tests. It was breaking tests from the `xmodule`
package on the devstack because the dist.key of its entry_points is `open-edx`.

xmodule/tests/xml/factories.py

@@ -11,7 +11,6 @@ from fs.osfs import OSFS
 from lxml import etree
 from xblock.mixins import HierarchyMixin
 
-from xmodule.modulestore import only_xmodules
 from xmodule.modulestore.inheritance import InheritanceMixin
 from xmodule.x_module import XModuleMixin
 
@@ -72,7 +71,6 @@ class XmlImportFactory(Factory):
 
     filesystem = OSFS(mkdtemp())
     xblock_mixins = (InheritanceMixin, XModuleMixin, HierarchyMixin)
-    xblock_select = only_xmodules
     url_name = Sequence(str)
     attribs = {}
     policy = {}