Mark the 1-per-sec update_current_time() call as redundant.

We still do this time update here, since we do it from all
callbacks, but it is no longer a reason to keep the once-per-second
callback enabled.

Closes ticket 26009.

changes/ticket26009

@@ -0,0 +1,9 @@
+  o Minor features (mainloop):
+   - Move responsibility for
+     keeping track of Tor's uptime
+      from a nce-per-second callback to a callback that is only scheduled as
+      needed.  Once enough items are removed from our once-per-second
+      callback, we can eliminate it entirely to conserve CPU when idle.
+      Closes ticket
+      26009.
+