cubicweb #2951066 remove async argument to call service [validation pending]

Call_service is able of both sync and sync execution making the API confusing. There is not user of the async case. We drop the async argument in favor of synchronous execution only.

This makes call_service the official API to call server side code from the client side. This is a remplacement for the usual monkey patching of the repo object.

done in3.19.0
load left0.000
closed by#614bf73cc126 [service] drop the asynchronous execution possibility
patchremove "async" flag from call_service (closes #2951066) [rejected]