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-fetcher-soup.c

@@ -49,7 +49,7 @@ typedef enum {
 } OstreeFetcherState;
 
 typedef struct {
-  volatile int ref_count;
+  int ref_count;  /* atomic */
 
   SoupSession *session;  /* not referenced */
   GMainContext *main_context;
@@ -77,7 +77,7 @@ typedef struct {
 } ThreadClosure;
 
 typedef struct {
-  volatile int ref_count;
+  int ref_count;  /* atomic */
 
   ThreadClosure *thread_closure;
   GPtrArray *mirrorlist; /* list of base URIs */