autoconvert: fix compiler warnings with g_atomic on recent GLib versions

autoconvert: fix compiler warnings with g_atomic on recent GLib versions

The volatile is not needed here and causes compiler warnings
with newer GLib versions.

gstautoconvert.c: In function ‘gst_auto_convert_dispose’ (and elsewhere):
glib/gatomic.h:108:3: warning: initialization discards ‘volatile’ qualifier from pointer target type [-Wdiscarded-qualifiers]
gstautoconvert.c:224:24: note: in expansion of macro ‘g_atomic_pointer_get’
  224 |     GList *factories = g_atomic_pointer_get (&autoconvert->factories);

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1237>