Fix cancel action on deletion confirmation form
Currently crashed with RequestError: invalid action '__action_cancel'.
It's broken as of https://hg.logilab.org/master/cubicweb/rev/cb217b2b3
This fix is similar as the one done in
https://hg.logilab.org/master/cubicweb/rev/9f1c89e7426d
Closes #13521493
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