Support polling git repositories for new branches/revisions

This is mostly a workaround for the occasional problems with the guix-commits
mailing list, as it can break and then the data service doesn't learn about
new revisions until the problem is fixed.

I think it's still a generally good feature though, and allows deploying the
data service without it consuming emails to learn about new revisions, and is
a step towards integrating some kind of way of notifying the data service to
poll.

guix-data-service/jobs/load-new-guix-revision.scm

@@ -344,22 +344,6 @@ WHERE job_id = $1")
        (simple-format #t "debug: Finished ~A, took ~A seconds\n"
                       action time-taken)))))
 
-(define (with-advisory-session-lock/log-time conn lock f)
-  (simple-format #t "debug: Acquiring advisory session lock: ~A\n" lock)
-  (let ((start-time (current-time)))
-    (with-advisory-session-lock
-     conn
-     lock
-     (lambda ()
-       (let ((time-taken (- (current-time) start-time)))
-         (simple-format #t "debug: Finished aquiring lock ~A, took ~A seconds\n"
-                        lock time-taken))
-       (let ((result (f)))
-         (let ((time-spent (- (current-time) start-time)))
-           (simple-format #t "debug: Releasing lock ~A, spent ~A seconds\n"
-                          lock time-spent))
-         result)))))
-
 (define (inferior-guix-systems inf)
   ;; The order shouldn't matter here, but bugs in Guix can lead to different
   ;; results depending on the order, so sort the systems to try and provide