Replace g_list_deep_free() with g_list_free_full()

Now that g_list_free_full() is in GLib we don't need to roll our own
version. Also use g_list_free_full() rather than g_list_foreach()
followed by g_list_free() to free a list and it's data.

https://bugzilla.gnome.org/show_bug.cgi?id=735091
Remove unneeded GLib version test

Now we require GLib 2.38 there is no need to test for older versions.

https://bugzilla.gnome.org/show_bug.cgi?id=736026