Remove more deprecated names.

[joaander]

Description

Remove msg_file and Gauss.

In #1497, I failed to notice that md.external.wall.Gauss. I doubt there are many users, so I will remove in v4 without a deprecation cycle in v3.

Motivation and context

See #1497.

How has this been tested?

I checked a local documentation build and did not see the removed quantities.

Change log

Added:

* ``hoomd.md.external.wall.Gaussian``

Removed:

* ``hoomd.md.pair.Gauss``
* ``hoomd.md.external.wall.Gauss``
* ``msg_file`` property and argument in ``hoomd.device.Device``.

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.

[b-butler]

Looks good.