[session] Kill support for a 'session_open' hook event
To be consistent with dropping of the 'session_close' event, and also because
this is already not called when using pyramid.
missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules)
embedded-javascript-library usr/share/cubicweb/cubes/shared/data/jquery.js