[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.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/html/index.html
Files: /usr/share/doc/cubicweb-documentation/html/*