[appimage thumbnailer] Rely on imported cmake target libappimage
Summary:
Recent released versions of libappimage set working imported cmake target,
and people better use the latest versions of libappimage anyway,
being a young & active project. No old widespread versions aronud
which need supporting here.
Test Plan: Appimage thumbnailer still builds and links (clean new build).
Reviewers: broulik, sitter
Reviewed By: sitter
Subscribers: sitter, kde-frameworks-devel, kfm-devel
Tags: Dolphin, Frameworks
Differential Revision: https://phabricator.kde.org/D18611