fix: rm xl.Dockerfile

This PR removes the xl.Dockerfile which is unused.

containers/jug/xl.Dockerfile

-#syntax=docker/dockerfile:1.4
-ARG DOCKER_REGISTRY="eicweb/"
-ARG BASE_IMAGE="jug_dev"
-ARG INTERNAL_TAG="testing"
-
-## ========================================================================================
-## STAGE1: spack builder image
-## EIC builder image with spack
-## ========================================================================================
-FROM ${DOCKER_REGISTRY}${BASE_IMAGE}:${INTERNAL_TAG}
-ARG TARGETPLATFORM
-
-## version will automatically bust cache for nightly, as it includes
-## the date
-ARG JUG_VERSION=1
-
-RUN echo " - jug_xl: ${JUG_VERSION}" >> /etc/jug_info