[hooks/syncschema] avoid altering a dropped table
If the constraint's rdef is being removed, CWConstraintDelOp doesn't
need to do anything. Otherwise it may try to alter a removed
table/column. Closes #4154549
missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/share/pyshared/cubicweb/skeleton/debian/rules.tmpl)