[NTP] Remove placeholder text in edit custom link URL field

Screenshot:
https://screenshot.googleplex.com/my3MWpB2rAv.png

Bug: 874194
Change-Id: Ifff7ae4f4e09c6a1c54c4c4215533ab0e1f78e4b
Reviewed-on: https://chromium-review.googlesource.com/1195811
Commit-Queue: Kristi Park <kristipark@chromium.org>
Commit-Queue: Mathieu Perreault <mathp@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587313}

chrome/browser/resources/local_ntp/custom_links_edit.html

@@ -24,8 +24,8 @@
       <div id="url" class="field-container">
         <label id="url-field-name" class="field-title"></label>
         <div class="input-container">
-          <input id="url-field" class="field-input" type="text" placeholder="https://"
-              autocomplete="off" spellcheck="false" tabindex="0" required></input>
+          <input id="url-field" class="field-input" type="text" autocomplete="off"
+              spellcheck="false" tabindex="0" required></input>
           <div class="underline"></div>
         </div>
         <div id="invalid-url" class="error-msg"></div>