[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