`pytest tests` no longer works so remove pytest plugins cruft

[robnagler]

___________________________________ ERROR collecting test session ____________________________________
Defining 'pytest_plugins' in a non-top-level conftest is no longer supported:
It affects the entire test suite instead of just below the conftest as expected.
  /home/vagrant/src/radiasoft/pykern/tests/pkcli/projex_work/proj1/tests/conftest.py
Please move it to a top level conftest file at the rootdir:
  /home/vagrant/src/radiasoft/pykern
For more information, visit:
  https://docs.pytest.org/en/stable/deprecations.html#pytest-plugins-in-non-top-level-conftest-files

There's a lot of cruft.

[robnagler]

Fixed in #456