cubicweb #2781782 drop hijack_user api on session [validation pending]

It semantic is dubious and it has very few user.

Most of its user could create a new session anyway Session(user, repo)

prioritynormal
typeenhancement
done in3.17.0
load0.250
load left0.000
closed by#48a6e6b88b16 [session] deprecate `hijack_user` method
patch[session] deprecate `hijack_user` method [applied]