maintner: fix data race in github sync

The lastUpdated field can only be used by the sync goroutine, but we
did labels and milestones concurrently in their own goroutines.

Remove the update because it was redundant with the caller's update
anyway.

Noticed in:
golang/go#19866 (comment)

Change-Id: I4b0270a00eaf2994ed096631e24a7bd75d0210ab
Reviewed-on: https://go-review.googlesource.com/42615
Reviewed-by: Kevin Burke <kev@inburke.com>

maintner/github.go

@@ -1335,7 +1335,6 @@ func (p *githubRepoPoller) syncMilestones(ctx context.Context) error {
 		return nil
 	}
 	p.c.addMutation(&maintpb.Mutation{Github: mut})
-	p.lastUpdate = time.Now()
 	return nil
 }
 
@@ -1368,7 +1367,6 @@ func (p *githubRepoPoller) syncLabels(ctx context.Context) error {
 	if changes == 0 {
 		return nil
 	}
-	p.c.addMutation(&maintpb.Mutation{Github: mut})
 	p.lastUpdate = time.Now()
 	return nil
 }