fix: revert "fix(toolbar): allow running Django tests runners..."

This reverts commit 22b83193.

While well intentioned, the original fix had some issues:

• The fix was to allow running poe manage test but the documented way to run tests is via one of poe test, poe tox:local or poe pytest:local which are all tested in CI. All of those three work because they use pytest as the test runner and IS_RUNNING_TESTS's default value detects this case.
• By hard-coding IS_RUNNING_TESTS to True, we are in fact lying for the other cases where settings.developer is used. Most notably those settings are used when running the application via poe up which is not when tests are being run.

The fact that the fix is not quite right has been highlighted in the original issue.

#129 has been opened to address the issue with running poe manage test.