datetime: 12h time format is now always available

Following a change in GDateTime, AM/PM format is now always available
even if the support is missing in the locale used.

Remove the code used to detect AM/PM support in the locale, and bump the
GLib requirements to the version that includes this GDateTime change.

See https://bugzilla.gnome.org/show_bug.cgi?id=761889

https://bugzilla.gnome.org/show_bug.cgi?id=780863