Remove centos:6 image references

This was EOL November 30th, 2020. I believe the "builds" are failing on
develop because of it.

lib/spack/docs/containers.rst

@@ -126,9 +126,6 @@ are currently supported are summarized in the table below:
    * - Ubuntu 18.04
      - ``ubuntu:18.04``
      - ``spack/ubuntu-bionic``
-   * - CentOS 6
-     - ``centos:6``
-     - ``spack/centos6``
    * - CentOS 7
      - ``centos:7``
      - ``spack/centos7``

lib/spack/spack/container/images.json

@@ -21,13 +21,6 @@
       "build_tags": {
         "develop": "latest"
       }
-    },
-    "centos:6": {
-      "os_package_manager": "yum",
-      "build": "spack/centos6",
-      "build_tags": {
-        "develop": "latest"
-      }
     }
   },
   "os_package_managers": {

lib/spack/spack/schema/container.py

@@ -12,8 +12,7 @@
             'type': 'string',
             'enum': ['ubuntu:18.04',
                      'ubuntu:16.04',
-                     'centos:7',
+                     'centos:7']
-                     'centos:6']
         },
         'spack': {
             'type': 'string',

share/spack/docker/centos-6.dockerfile

@@ -1,74 +0,0 @@
-FROM centos:6
-MAINTAINER Spack Maintainers <maintainers@spack.io>
-
-ENV DOCKERFILE_BASE=centos            \
-    DOCKERFILE_DISTRO=centos          \
-    DOCKERFILE_DISTRO_VERSION=6       \
-    SPACK_ROOT=/opt/spack             \
-    DEBIAN_FRONTEND=noninteractive    \
-    CURRENTLY_BUILDING_DOCKER_IMAGE=1 \
-    container=docker
-
-# Make yum usable again with CentOS 6
-RUN curl https://www.getpagespeed.com/files/centos6-eol.repo --output /etc/yum.repos.d/CentOS-Base.repo
-
-RUN yum update -y \
- && yum install -y epel-release \
- && yum update -y \
- && yum --enablerepo epel groupinstall -y "Development Tools" \
- && yum --enablerepo epel install -y \
-        curl \
-        findutils \
-        gcc-c++ \
-        gcc \
-        gcc-gfortran \
-        git \
-        gnupg2 \
-        hostname \
-        iproute \
-        Lmod \
-        make \
-        patch \
-        python \
-        python-pip \
-        python-setuptools \
-        tcl \
-        unzip \
-        which \
- && rm -rf /var/cache/yum \
- && yum clean all
-
-COPY bin   $SPACK_ROOT/bin
-COPY etc   $SPACK_ROOT/etc
-COPY lib   $SPACK_ROOT/lib
-COPY share $SPACK_ROOT/share
-COPY var   $SPACK_ROOT/var
-RUN mkdir -p $SPACK_ROOT/opt/spack
-
-RUN ln -s $SPACK_ROOT/share/spack/docker/entrypoint.bash \
-          /usr/local/bin/docker-shell \
- && ln -s $SPACK_ROOT/share/spack/docker/entrypoint.bash \
-          /usr/local/bin/interactive-shell \
- && ln -s $SPACK_ROOT/share/spack/docker/entrypoint.bash \
-          /usr/local/bin/spack-env
-
-RUN mkdir -p /root/.spack \
- && cp $SPACK_ROOT/share/spack/docker/modules.yaml \
-        /root/.spack/modules.yaml \
- && rm -rf /root/*.* /run/nologin $SPACK_ROOT/.git
-
-# [WORKAROUND]
-# https://superuser.com/questions/1241548/
-#     xubuntu-16-04-ttyname-failed-inappropriate-ioctl-for-device#1253889
-RUN [ -f ~/.profile ]                                               \
- && sed -i 's/mesg n/( tty -s \\&\\& mesg n || true )/g' ~/.profile \
- || true
-
-WORKDIR /root
-SHELL ["docker-shell"]
-
-# TODO: add a command to Spack that (re)creates the package cache
-RUN spack spec hdf5+mpi
-
-ENTRYPOINT ["/bin/bash", "/opt/spack/share/spack/docker/entrypoint.bash"]
-CMD ["interactive-shell"]