I've been playing around with Gtk and vala and I love it; however, I've created two simple apps that I don't know how to delete from my system.
If you still have the source code, use
sudo ninja uninstall inside the build folder.
If you removed the build folder by any chance, run
meson build --prefix=/usr to recreate the build folder, followed by
cd build && ninja.
If you no longer have the source code, you'll have to manually remove your binary from
/usr/bin/ and the .desktop file from
/usr/share/applications/. Also, look inside
/usr/share/metainfo/ for any xml files.