cubicweb #3515223 [editcontroller] Remove 3.13 incomplete backward compat [validation pending]

regarding _cw_entity_fields (it is present for only one usage, thus breaks anyway).

prioritynormal
typeenhancement
done in3.19.0
load0.100
load left0.000
closed by#78702b55c089 Drop 3.13 incomplete backward compat in edit controller.
patchDrop 3.13 incomplete backward compat in edit controller. [applied]