tests: Drop screenshots from metainfo

[bbhtt]

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

[bbhtt]

nevermind...

[bbhtt]

ah looks like it's appstream somehow sometimes getting stuck.

2026-03-10T15:37:36.4005207Z Running appstreamcli compose
2026-03-10T15:37:36.4005730Z Saving screenshots in /var/tmp/test-flatpak-5zTxki/.fp-compose-url-policy-full/rofiles/rofiles-rz1f56/files/share/app-info/media
2026-03-10T15:37:36.4006452Z Only accepting components: org.flatpak.appstream_media, org.flatpak.appstream_media.desktop
2026-03-10T15:37:36.4007103Z Processing directory: /var/tmp/test-flatpak-5zTxki/.fp-compose-url-policy-full/rofiles/rofiles-rz1f56/files
2026-03-10T15:37:36.4007722Z Composing metadata...
2026-03-10T15:37:36.4007909Z OK closing connection
2026-03-10T15:37:36.4008077Z Terminated
2026-03-10T15:37:36.4008526Z rm: cannot remove '/var/tmp/test-flatpak-5zTxki/.fp-compose-url-policy-full/rofiles/rofiles-rz1f56': Device or resource busy