[webrequest] simplify set_session code
Thanks to the previous changeset we are assured that session handed to
set_session is full featured one. This allows a simpler code for this method.
related to #2503918
debian/tmp/usr/bin/cubicweb-ctl usr/bin/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/cwctl.py usr/lib/PY_VERSION/site-packages/cubicweb
debian/cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl