First pass at removing deprecated functionality.

[joaander]

Description

Remove:

• LLVM
• box1/box2/variant arguments to BoxResize
• from_gsd_snapshot

Motivation and context

Remove deprecated features in preparation for a HOOMD-blue 5.0 release.

This is the first batch. Later PRs will remove TBB and depletants.

How has this been tested?

CI checks.

Change log

Removed:

* ``Snapshot.from_gsd_snapshot``
  (`#1888 <https://github.com/glotzerlab/hoomd-blue/pull/1888>`__).
* ``box1``, ``box2``, and ``variant`` arguments to ``hoomd.update.BoxResize``
  (`#1888 <https://github.com/glotzerlab/hoomd-blue/pull/1888>`__).
* ``hoomd.util.GPUNotAvailableError``
  (`#1888 <https://github.com/glotzerlab/hoomd-blue/pull/1888>`__).
* ``hpmc.pair.user.CPPPotentialBase``, ``hpmc.pair.user.CPPPotential``,
  ``hpmc.pair.user.CPPPotentialUnion``, and ``hpmc.integrate.HPMCIntegrator.pair_potential``
  (`#1888 <https://github.com/glotzerlab/hoomd-blue/pull/1888>`__).
* ``hoomd.hpmc.external.user.CPPExternalPotential``
  (`#1888 <https://github.com/glotzerlab/hoomd-blue/pull/1888>`__).

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the HOOMD-blue Contributor Agreement.
• My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.