[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.

pytest
http://hg.logilab.org/master/logilab/common/archive/default.tar.bz2#egg=logilab-common
http://hg.logilab.org/master/yams/archive/default.tar.bz2#egg=yams