testing/lsp-plugins: remove unneeded PREFIX, set xdg subpackage to noarch

Following up on a comment from the upstream developer about the PREFIX variable which I missed when it was originally posted: https://github.com/sadko4u/lsp-plugins/issues/279#issuecomment-1329012001

Also, I think ::noarch for the xdg subpackage is ok here to silence the warning during packaging step.