epa-key-list-mode-map: Use widget-keymap as parent keymap

epa-key-list-mode-map: Use widget-keymap as parent keymap

Normally when one keymap is to be treated as the parent of another,
then that relationship is setup once at the time when the child is
being defined, i.e. at birth.  For some reason, this was not done
here; instead `widget-keymap' is set as `epa-key-list-mode-map'
parent every time the former is setup to be used as the local map.

This appears to be a mistake.  A few other keymaps use `widget-keymap'
as their parent and in those cases the relationship is established
just once. `epa-key-list-mode-map' is the only exception and because
there is absolutely no indication that that is justified, we remove
this inconsistency.

* lisp/epa.el (epa-key-list-mode-map): Set the parent of this keymap
while defining it.
* lisp/epa.el (epa--list-keys): Do not set the parent of the local
keymap here.