Remove config extra from tox.ini

The `config` extra is defined is none of our e3 `pyproject.toml` files.
And an error is now raised by tox for unknown extras since version
v4.36.0 (2026-02-15)

ref it/cookiecutter-e3#105
ref it/e3-core#6

See merge request it/cookiecutter-e3!102 of the template

Author: cookiecutter-e3

tox.ini

@@ -34,7 +34,6 @@ commands=
 
 [testenv:check]
 extras =
-    config
     check
 commands =
 # Run bandit checks. Accept yaml.load(), pickle, and exec since this