[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.

Release notes
-------------

.. toctree::
    :maxdepth: 1

    3.25
    3.24
    3.23
    3.22
    3.21
    3.20
    3.19
    3.18
    3.17
    3.16
    3.15
    3.14