Remove zero-width space from AppStream metadata

When copying an appdata.xml template, a zero-width space got into the file, but it wasn't shown in my editor, nor in Git, nor on GitLab. Really weird o_O

sed -i 's/\xe2\x80\x8b//g' debian/data/org.fdroid.repomaker.appdata.xml

https://en.wikipedia.org/wiki/Zero-width_space

I found this when trying to build the flatpak. I now tested it with the version of this MR and it works.