cubicweb #2919299 deprecate old multi-source [validation pending]

We moved to the feed approached. dropping multi source is expected in 3.19.0

prioritynormal
typeenhancement
done in3.18.0
load0.500
load left0.000
closed by#1236d9058ad3 [pyrorql] deprecate pyrorql source (closes #2919299), #af36f459d2f0 [server] deprecate the old multi-source system
patch[pyrorql] deprecate pyrorql source (closes #2919299) [applied][server] deprecate the old multi-source system [applied]