[NTP] Change size of mic icon in fakebox

Latest UI/UX decision is to revert to the previous size for the mic icon.

Bug: 859383
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I147a11890112dbc78063254693cbe6bb31bc34b1
Reviewed-on: https://chromium-review.googlesource.com/1122971Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572213}

chrome/browser/resources/local_ntp/local_ntp.css

@@ -406,7 +406,6 @@ html[dir=rtl] #fakebox-cursor {
 }
 
 .md #fakebox-microphone {
-  background-size: 16px 16px;
   margin-right: 12px;
   width: 28px;
 }