dashboard: fix update script

It needs to remove perf results rather than build results.

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/164000043

dashboard/app/build/update.go

@@ -58,7 +58,7 @@ func updateBenchmark(w http.ResponseWriter, r *http.Request) {
 				continue
 			}
 			com.NeedsBenchmarking = true
-			com.ResultData = nil
+			com.PerfResults = nil
 			if err := putCommit(c, com); err != nil {
 				return err
 			}