[ntp] Fix enum numbering for NTPBackgroundImageSource.

The current numbering for the enum is not in sync with what exists at chrome/browser/ui/webui/new_tab_page/new_tab_page.mojom. We need to remove an entry from enums.xml or add an entry to the mojom. Since image sources are always set when they are used we should remove the unused enum.

Verified locally using chrome://histograms

Change-Id: If3b237265576b3afbe42075809be1d52ffda9890
Bug: 1384258
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4201000
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Paul Adedeji <pauladedeji@google.com>
Cr-Commit-Position: refs/heads/main@{#1098205}

tools/metrics/histograms/enums.xml

@@ -73594,7 +73594,6 @@ Called by update_net_trust_anchors.py.-->
 </enum>
 
 <enum name="NTPBackgroundImageSource">
-  <int value="0" label="Unknown source"/>
   <int value="1" label="No image"/>
   <int value="2" label="First party (NTP) theme without refresh daily enabled"/>
   <int value="3" label="First party (NTP) theme with refresh daily enabled"/>