This reverts commit 80fcb1bc.

G_DISABLE_DEPRECATED should never be used by anybody, least of all by
GLib. We have deprecation annotations for the compiler, these days, and
they are much better suited than a macro that makes symbols appear and
disappear. The fact that gtk-doc doesn't understand the deprecation
annotations is a limitation of gtk-doc, and it's gtk-doc that ought to be
fixed.

Commit 80fcb1bc broke GStreamer, which disables old API that was
deprecated before the introduction of the deprecation annotations, but
still uses newly deprecated one, and relies on the deprecation
annotations to do their thing. It also broke libsoup, as it uses
GValueArray in its own API.