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: https://github.com/openedx/XBlock/issues/714
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`.
- Moving xmodule folder to root as we're dissolving sub-projects of common folder in edx-platform
- More info: https://openedx.atlassian.net/browse/BOM-2579
- -e common/lib/xmodule has been removed from the requirements as xmodule has itself become the part of edx-platform and not being installed through requirements
- The test files common/lib/xmodule/test_files/ have been removed as they are not being used anymore
