[session] drop .is_repo_in_memory This is deader than a dead dog's bone buried down a blind alley off a dead-end street in a ghost town. Related to #1381328.

usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py