Follow-up to 7220e47a

Remove app_modal_strings.grdp, which was left behind as a workaround
for a presubmit bug.

Bug: 1047487
Change-Id: Idd906bc9d1ec127f442925f0399af41911064d1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033696
Commit-Queue: Evan Stade <estade@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Auto-Submit: Evan Stade <estade@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737513}

components/app_modal_strings.grdp

-<?xml version="1.0" encoding="utf-8"?>
-<grit-part>
-
-<!-- Presubmit won't let this file be deleted. -->
-
-</grit-part>