refactor!: remove safe_lxml import compatibility hack (#30671)

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.

safe_lxml/__init__.py

@@ -1,36 +0,0 @@
-"""
-Temporary import path shim module for openedx.core.lib.safe_lxml.
-
-Previously, the safe_lxml package was housed in common/lib/safe_lxml.
-It was installed as its own Python project, so instead of its import path
-being, as one would expect:
-
-    import common.lib.safe_lxml.safe_lxml
-
-it was instead just:
-
-    import safe_lxml
-
-To increase the sanity of edx-platform and simplify its tooling, we have
-moved the safe_lxml package to openedx/core/lib (in tihs same repo) and
-changing its import path to:
-
-    import openedx.core.lib.safe_lxml
-
-For details, see:
-https://openedx.atlassian.net/browse/BOM-2583 (public, but requires account)
-
-In order to maintain backwards-compatibility with code using the
-old import path for one release, we expose this temporary compatibility
-module and raise a warning when the old import path is used.
-This entire `safe_lxml` shim folder can be removed after 2023-01-01.
-In the Poplar release, 'import safe_lxml' will stop working entirely.
-"""
-import warnings
-
-warnings.warn(
-    "Importing from 'safe_lxml' instead of 'openedx.core.lib.safe_lxml' is deprecated.",
-    stacklevel=3,  # Should surface the line that is doing the importing.
-)
-
-from openedx.core.lib.safe_lxml import *  # pylint: disable=unused-wildcard-import,wrong-import-order