Android: Delete unused drawable "ic_snippet_thumbnail_placeholder.xml"

Wasn't quite unused - but was used only for Android Studio's layout
previewer. I don't know of a good way to have non-shipped resources,
so deleting is probably the best option for now.

Bug: 988047
Change-Id: Ic002ed2cbf2408100b70110436c01a9922a0fb03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1749705
Commit-Queue: Theresa  <twellington@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686160}

chrome/android/java/res/drawable/ic_snippet_thumbnail_placeholder.xml

-<?xml version="1.0" encoding="utf-8"?>
-<!-- Copyright 2016 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. -->
-<!-- TODO(https://crbug.com/602627) tools:targetApi added to mute the NewApi lint error. -->
-<vector xmlns:android="http://schemas.android.com/apk/res/android"
-    xmlns:tools="http://schemas.android.com/tools"
-    tools:targetApi="21"
-    android:width="@dimen/snippets_thumbnail_size"
-    android:height="@dimen/snippets_thumbnail_size"
-    android:viewportWidth="72"
-    android:viewportHeight="72">
-
-    <path android:fillColor="@color/modern_secondary_color"
-        android:pathData="M0,0 L72,0 L72,71.9997 L0,71.9997 Z" />
-    <path android:fillColor="@color/default_icon_color"
-        android:pathData="M45,42.9997 L45,28.9997 C45,27.8997,44.1,26.9997,43,26.9997 L29,26.9997
-C27.9,26.9997,27,27.8997,27,28.9997 L27,42.9997
-C27,44.0997,27.9,44.9997,29,44.9997 L43,44.9997
-C44.1,44.9997,45,44.0997,45,42.9997 Z M32.5,37.4997 L35,40.5097 L38.5,35.9997
-L43,41.9997 L29,41.9997 L32.5,37.4997 Z" />
-</vector>

chrome/android/java/res/layout/content_suggestions_card_modern_reversed.xml

@@ -108,8 +108,7 @@
             android:layout_alignParentEnd="true"
             android:scaleType="centerCrop"
             tools:ignore="ContentDescription"
-            android:src="@null"
-            tools:src="@drawable/ic_snippet_thumbnail_placeholder" />
+            android:src="@null" />
     </RelativeLayout>
 
     <!-- It would have been nice to use RoundedBitmapDrawable on the thumbnail, but with that you