[server/session] remove json dumps around call_service
This backouts changeset b151beea9cb6. With call_service being
in-process and synchronous, there's no RPC use case.
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/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html