Remove no longer supported centos6 distrib test

tools/dockerfile/distribtest/ruby_centos6_x64/Dockerfile

@@ -1,37 +0,0 @@
-# Copyright 2015 gRPC authors.
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-FROM centos:6
-
-RUN yum install -y curl
-
-RUN yum install -y tar which
-
-# Install rvm
-RUN gpg --keyserver hkp://keys.gnupg.net --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3 7D2BAF1CF37B13E2069D6956105BD0E739499BDB
-RUN curl -sSL https://get.rvm.io | bash -s stable
-
-# Install Ruby 2.3
-# Running the installation twice to work around docker issue when using overlay.
-# https://github.com/docker/docker/issues/10180
-RUN (/bin/bash -l -c "rvm install ruby-2.3.8") || (/bin/bash -l -c "rvm install ruby-2.3.8")
-RUN /bin/bash -l -c "rvm use --default ruby-2.3.8"
-RUN /bin/bash -l -c "echo 'gem: --no-document' > ~/.gemrc"
-RUN /bin/bash -l -c "echo 'export PATH=/usr/local/rvm/bin:$PATH' >> ~/.bashrc"
-RUN /bin/bash -l -c "echo 'rvm --default use ruby-2.3.8' >> ~/.bashrc"
-RUN /bin/bash -l -c "gem install bundler -v 1.17.3 --no-document"
-
-RUN mkdir /var/local/jenkins
-
-RUN /bin/bash -l -c "echo '. /etc/profile.d/rvm.sh' >> ~/.bashrc"

tools/run_tests/artifacts/distribtest_targets.py

@@ -369,7 +369,6 @@ def targets():
                         'jessie',
                         ruby_version='ruby_2_3',
                         source=True),
-        RubyDistribTest('linux', 'x64', 'centos6'),
         RubyDistribTest('linux', 'x64', 'centos7'),
         RubyDistribTest('linux', 'x64', 'fedora23'),
         RubyDistribTest('linux', 'x64', 'opensuse'),