Revert "[ios] Set AppIcon as default intent image."

This reverts commit ddae444d.

Reason for revert: A quick survey of other app store apps shows no one
else is setting the INIntentDefaultImageName value.  It's possible the
'ghost icon' won't appear when the app comes from the app store.  And
when we do set INIntentDefaultImageName, the image gets shown alongside
 the app icon (which is identical) leading to two chrome icons showing
 up twice.

Siri Shortcuts are broken regardless if you delete the app, so I think
showing the ghost icon is 'ok'.

Original change's description:
> [ios] Set AppIcon as default intent image.
>
> Bug: 916994
> Change-Id: I93d0e6264e842fb445c30b5b73396695823ade6c
> Reviewed-on: https://chromium-review.googlesource.com/c/1387044
> Reviewed-by: Peter Lee <pkl@chromium.org>
> Commit-Queue: Justin Cohen <justincohen@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#618248}

Bug: 916994, 920189
Change-Id: I1113f7638d4a29d9c20d2aa1ddb91030ce4432d3
Reviewed-on: https://chromium-review.googlesource.com/c/1406164Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621958}

ios/chrome/app/intents/Intents.intentdefinition

@@ -17,8 +17,6 @@
 		<dict>
 			<key>INIntentCategory</key>
 			<string>information</string>
-			<key>INIntentDefaultImageName</key>
-			<string>AppIcon</string>
 			<key>INIntentDescriptionID</key>
 			<string>k0Ho6W</string>
 			<key>INIntentLastParameterTag</key>