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
[MESSAGES CONTROL]
disable-msg = C0301
[VARIABLES]
# Enable / disable this checker
enable-variables = yes
additional-builtins = _, display_name
[BASIC]
required-attributes=
attr-rgx = (w|[a-z_][a-z0-9_]{2,30})
variable-rgx = (w|[a-z_][a-z0-9_]{2,30})$
argument-rgx = (w|[a-z_][a-z0-9_]{2,30})$
[TYPECHECK]
generated-members=debug,info,notice,warning,error,critical,exception