Use floating centos tag and remove EOL RHEL Dockerfile

eng/docker/centos/Dockerfile

@@ -4,7 +4,7 @@
 #
 
 # Dockerfile that creates a container suitable to build dotnet-cli
-FROM mcr.microsoft.com/dotnet-buildtools/prereqs:centos-7-b46d863-20180719033416
+FROM mcr.microsoft.com/dotnet-buildtools/prereqs:centos-7
 
 RUN yum -q -y install sudo
 

eng/docker/rhel/Dockerfile

@@ -1,27 +0,0 @@
-#
-# Copyright (c) .NET Foundation and contributors. All rights reserved.
-# Licensed under the MIT license. See LICENSE file in the project root for full license information.
-#
-
-# Dockerfile that creates a container suitable to build dotnet-cli
-FROM mcr.microsoft.com/dotnet-buildtools/prereqs:rhel-7-rpmpkg-e1b4a89-20175311035359
-
-# Setup User to match Host User, and give superuser permissions
-ARG USER_ID=0
-RUN useradd -m code_executor -u ${USER_ID} -g root
-RUN echo 'code_executor ALL=(ALL) NOPASSWD:ALL' >> /etc/sudoers
-
-# With the User Change, we need to change permissions on these directories
-RUN chmod -R a+rwx /usr/local
-RUN chmod -R a+rwx /home
-RUN chown root:root /usr/bin/sudo && chmod 4755 /usr/bin/sudo
-
-# Set user to the one we just created
-USER ${USER_ID}
-
-# Set working directory
-ARG WORK_DIR
-WORKDIR ${WORK_DIR}
-
-# Set up Azure Artifacts credential provider
-RUN wget -qO- https://raw.githubusercontent.com/Microsoft/artifacts-credprovider/master/helpers/installcredprovider.sh | bash