windowManager: Replace sessionMode.allowKeybindingsWhenModal

The original condition the property was based on was added to make
the a11y switcher available in the login screen, though it did never
work properly - after popping up the switcher, additional tab key
presses were ignored. As we are now able to filter bindings much more
selectively, we can simplify the check and drop the sessionMode property.

https://bugzilla.gnome.org/show_bug.cgi?id=688202