Drop unused tox:jenkins section

This is unused by the jenkins-job config on
https://jenkins.logilab.org/job/cubicweb-blog/ because we call the py27 and
flake8-jenkins environments directly.

tox.ini

 [tox]
 envlist = py27,flake8
 
-[tox:jenkins]
-envlist = py27, flake8-jenkins
-
 [testenv]
 sitepackages = True
 deps =