Remove redundant "existing page" pane configuration

The "Existing page" pane configuration is plain redundant:
* if used inside the overridden page the info about the page to render comes from the menu call.
* the "existing page" info is *only* used to get the title of the page which should be done by setting page title
* this page title should default to the page title from menu (which can be overridden in pane config anyway)
* which makes the config "Use the menu title for the pane title" also redundant

To not break existing titles we might set page title accordingly with a little autoupdate code.

See also #2187205: Include overridden page on other pages.

I might code this (and help maintain if wished) if OK by maintainer @swentel.