cubicweb #5334750 reledit shouldn't propose to drop existing relation if it's mandatory [validation pending]

it currently doesn't check for the relation's cardinality

prioritynormal
typebug
done in3.20.8
load0.500
load left0.000
closed by#9222e056584e [reledit] also check for cardinality before displaying the delete relation action. Closes #5334750
patch[reledit] also check for cardinality before displaying the delete relation action. Closes #5334750 [applied]