keybindings: Remove special-case code for reversing automatically

Since we now directly expose the reverses bindings directly, we
don't have to have this special-case in do_choose_window.

More importantly, if the backwards binding is pressed and has the Shift
key included, this will actually revert it

This doesn't matter for Alt-Tab in gnome-shell, which already replaces
it with a better Alt-Tab replacement, but it does matter for Alt-Esc,
which switches between windows directly.