cubicweb #2758254 remove leftover uses of deprecated JSONController [validation pending]

Follow up to ticket #2110265, there are still uses of the old /json URL.

prioritynormal
typetask
done in3.15.10
load left0.000
closed by#75be9de9d68e [web] Use the new '/ajax' URL path to access the AjaxController (closes #2758254)
patch[web] Use the new '/ajax' URL path to access the AjaxController [applied]