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-repo-private.h

@@ -72,7 +72,7 @@ typedef enum {
 } OstreeRepoTestErrorFlags;
 
 struct OstreeRepoCommitModifier {
-  volatile gint refcount;
+  gint refcount;  /* atomic */
 
   OstreeRepoCommitModifierFlags flags;
   OstreeRepoCommitFilter filter;