combobox: Remove dead code & unneeded type checks

combobox: Remove dead code & unneeded type checks

Commit fdc0c6426b6fb751653f3a8536a344f69a6407ac removed the appears-as-
list style property, & hence the ability to put the ComboBox into list
mode – but it left behind a pile of hijinks that were only used in said
mode & so were now doing absolutely nothing. This commit deletes those.

While doing that, I got carried away…so this also stops pointlessly type
checking popup_widget, as that can never be anything but a GtkTreeMenu.
It still checks for NULL everywhere, which shouldn’t be needed, but (A)
this commit is already too big, & (B) simply removing such checks where
they _seem_ unnecessary causes bad times. I’ll puzzle through that later