optimize-png-files.sh: drop -i flag from bash invocation

Running bash in interactive mode doesn't make much sense, especially
when this script is not interactive.  The commit that added didn't
note why, but looks like it might have been an accident.  Drop it.

Bug: None
Change-Id: I491e31e7e411d48a5e527793d9889b81733a2ccb
Reviewed-on: https://chromium-review.googlesource.com/1096081
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566330}

tools/resources/optimize-png-files.sh

@@ -1,4 +1,4 @@
-#!/bin/bash -i
+#!/bin/bash
 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.