refactor!: remove safe_lxml import compatibility hack

[kdmccormick]

Description

In the past, safe_lxml was located at ./common/lib/safe_lxml/safe_lxml,
and was imported as so:

  import safe_lxml

Now, safe_lxml is located at ./openedx/core/lib/safe_lxml, and is
imported as so:

  import openedx.core.lib.safe_lxml

We added in a temporary backwards-compatibility hack, in the form
of the module ./safe_lxml/__init__.py, to support the old import
format (with a warning written to the logs).

Enough time has passed that it we feel it is safe to
remove that compatibility hack. After this commit, imports in
the form `import safe_lxml` will raise an ImportError.

Supporting information

• PR that changed the safe_lxml import path: refactor: make safe_lxml an ordinary folder in openedx/core/lib  #25689
• Ticket describing that change: https://openedx.atlassian.net/browse/BOM-2583
• Epic ticket describing the reasoning for the change: https://openedx.atlassian.net/browse/BOM-2579
• MIT ODL edx-sga PR to fix old usage of import safe_lxml: fix!: use full import path to safe_lxml mitodl/edx-sga#324

Testing instructions

edX/2U: Please search the LMS and CMS logs for the string: Importing from 'safe_lxml' instead of 'openedx.core.lib.safe_lxml' is deprecated.. There should be zero occurrences for the past month. If there are occurances, then we need to address those before merging this PR.

Deadline

None, although we should wrap this up before considering BOM-2579 "done".

Other information

Blocked until:

• fix!: use full import path to safe_lxml mitodl/edx-sga#324 merges
• edx-sga version 0.20.0 is released (should be within a day or two)
• Requirement bot updates edx-sga pin in edx-platform to 0.20.0
• edx.org release happens
• 2U checks logs again for safe_lxml warnings

[kdmccormick]

@awais786 says that the last log relevant edX.org message was:

Sep 13 09:02:59 ip-x-x-x-x [service_variant=lms][py.warnings][env:prod-edx-edxapp] WARNING [ip-x-x-x-x] [user None] [ip None] [warnings.py:109] - /edx/app/edxapp/venvs/edxapp/lib/python3.8/site-packages/pkg_resources/__init__.py:2453: UserWarning: Importing from 'safe_lxml' instead of 'openedx.core.lib.safe_lxml' is deprecated.

Which is about an hour after the upgrade to edx-sga==0.20.0 (which incorporates mitodl/edx-sga#324) was deployed to edX production.

So, I believe that this is safe to merge.

[edx-pipeline-bot]

EdX Release Notice: This PR has been deployed to the staging environment in preparation for a release to production.

[edx-pipeline-bot]

EdX Release Notice: This PR has been deployed to the production environment.

[edx-pipeline-bot]

EdX Release Notice: This PR has been deployed to the production environment.