[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.

mock
psycopg2
ldap3 < 2
cubicweb-basket
cubicweb-card
cubicweb-comment
cubicweb-file
cubicweb-localperms
cubicweb-tag