Remove menu->menu hack when redirecting pager ops through index.

Remove menu->menu hack when redirecting pager ops through index.

As noted in commit a327386c, this causes problems when redirecting ops
that open new menus.  There is no need to change the menu->menu type,
as a flag works perfectly well with no such side effects.

This also removes the need for comments explaning the hack before
mutt_folder_hooks() when changing folders, or before displaying a
message in the pager.