cubicweb #4154549 migration crash on dropping an etype [validation pending]

when dropping an etype, an operation is scheduled to handle constraints going away, which crashes trying to access the dropped table

prioritynormal
typebug
done in3.19.3
load0.500
load left0.000
closed by#5b48dcc02be1 [hooks/syncschema] avoid altering a dropped table
patch[hooks/syncschema] avoid altering a dropped table [applied]