ci: Remove print-backends build option

Flatpak CI is failing because of unknown option "print-backends".
print-backends was renamed to print in c4d350c260
and subsequently was removed in a4aa6d79ad
(replaced by print-cups and print-cloudprint as auto options)

.gitlab-ci/flatpak-build.sh

@@ -19,7 +19,6 @@ flatpak build ${builddir} meson \
                 --buildtype=release \
                 -Dx11-backend=true \
                 -Dwayland-backend=true \
-                -Dprint-backends=file \
                 -Dbuild-tests=false \
                 -Dbuild-examples=false \
                 -Dintrospection=disabled \