glib-2.0: remove HashTable.get_keys_as_array
glib-2.0: remove HashTable.get_keys_as_array

This was causing a syntax error when trying to compile Vala with an
older version of valac (whatever is in gnome-continuous).  It looks
like we should hold off on adding stuff with this syntax until
next cycle.

rvu95/vala,spidermonkdust/vala,frida/vala,frida/vala,GNOME/vala,spidermonkdust/vala,frida/vala,frida/vala,flobrosch/vala-fixes,GNOME/vala,flobrosch/vala-fixes,GNOME/vala,frida/vala,rvu95/vala,GNOME/vala,rvu95/vala,spidermonkdust/vala,flobrosch/vala-fixes