Drop use of volatile

As detailed in
gitlab.gnome.org/GNOME/glib/-/issues/600#note_877282, volatile
isn't actually needed in these contexts because the atomic operations
already give us strong enough guarantees. In GCC 11, this triggers a
diagnostic due to the volatile qualifier getting dropped anyway.

There is a WIP to do the same in glib:
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1719

This obsoletes this downstream patch:
https://src.fedoraproject.org/rpms/ostree/c/b8c5a6fb

src/libostree/ostree-remote-private.h

@@ -41,7 +41,7 @@ G_BEGIN_DECLS
  * remote which this one inherits from, and is what should be used in refspecs
  * for pulls from this remote. If it’s %NULL, @name should be used instead. */
 struct OstreeRemote {
-  volatile int ref_count;
+  int ref_count;  /* atomic */
   char *name;  /* (not nullable) */
   char *refspec_name;  /* (nullable) */
   char *group;   /* group name in options (not nullable) */