appstream: Drop flatpak_dir_check_for_appstream_update

This early bailout is not really needed, because noop updates is
pretty fast. Also, doing that breaks the timestamp updates.

Closes: #1585
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f