flatpak-build.sh: Do not disable vulkan

In a94d7abf0d we removed
the explicit disable from the manifest.

Now we should also have the CI build the vulkan render as well.

.gitlab-ci/flatpak-build.sh

@@ -19,7 +19,6 @@ flatpak build ${builddir} meson \
                 --buildtype=debugoptimized \
                 -Dx11-backend=true \
                 -Dwayland-backend=true \
-                -Dvulkan=disabled \
                 -Dbuild-tests=false \
                 -Dbuild-testsuite=false \
                 -Dbuild-examples=false \