tests: Drop screenshots from metainfo

This aren't required since the test is checking for the presence
of a mirrored icon link. Appstream tries to download screenshots
if present and ignores download failures anyway silently. This may be
contributing to the flakiness in [1]. Since the test already works
offline, the screenshot links are redundant

[1]: #714 (comment)

Author: bbhtt

tests/org.flatpak_builder.gui.metainfo.xml

@@ -11,22 +11,6 @@
   <description>
     <p>An example desktop application</p>
   </description>
-  <screenshots>
-    <!-- appstream silently continues without error if the file fails to
-         download. since the tag only accepts http(s) urls, we add
-         multiple sources so that in case one link fails, at least one
-         of the other ones work. Replace with any other screenshot
-         link if it fails
-     -->
-    <screenshot type="default">
-      <caption>An example screenshot</caption>
-      <image>https://raw.githubusercontent.com/flatpak/flatpak.github.io/0b56895e271bbcc7f86f9570933a96adff99e110/source/img/endless-apps.original.png</image>
-    </screenshot>
-    <screenshot>
-      <caption>An example screenshot</caption>
-      <image>https://docs.flathub.org/img/card.png</image>
-    </screenshot>
-  </screenshots>
   <url type="bugtracker">https://flatpak.org</url>
   <content_rating type="oars-1.1"/>
   <releases>