Drop RHEL6 references

Fixes: https://pagure.io/koji/issue/3165

devtools/containers/Dockerfile.centos6

@@ -1,28 +0,0 @@
-FROM centos:6
-RUN \
-  yum install -y \
-    gcc \
-    git \
-    make \
-    krb5-devel \
-    libffi-devel \
-    openssl-devel \
-    pyOpenSSL \
-    python-cheetah \
-    python-devel \
-    python-requests \
-    redhat-rpm-config \
-    rpm-build \
-    rpm-python \
-    yum-utils && \
-  yum install -y epel-release && \
-  yum install -y \
-    python-flake8 \
-    python-multilib \
-    python-pip \
-    python-psycopg2 \
-    python-qpid-proton \
-    python-requests-kerberos \
-    yumdownloader && \
-  pip install -U 'pip==9.0.1' && \
-  pip install -U tox

devtools/containers/README.md

@@ -4,7 +4,6 @@ Dockerfiles for development
 To facilitate in development - specifically, running tests, some Dockerfiles are
 provided:
 
-* [`./Dockerfile.centos6`](./Dockerfile.centos6) CentOS 6, for testing with python2.6
 * [`./Dockerfile.centos7`](./Dockerfile.centos7) CentOS 7, for testing with python2.7
 * [`./Dockerfile.centos8`](./Dockerfile.centos8) CentOS 8, for testing with python3.6
 * [`./Dockerfile.f32`](./Dockerfile.f32) Fedora 32, for testing with python3.8