misc: Removed python3 Dockerfile

This was previously used to test gem5 being compiled and run in a
Python3 environment. This is redundant with the introduction of
"util/dockerfiles/ubuntu-20.04_all-dependencies", which uses python3
exclusively.

Change-Id: Ie837da338c3985ba92aff84144948a23fd6ece3f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28890
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/util/dockerfiles/python3/Dockerfile b/util/dockerfiles/python3/Dockerfile
deleted file mode 100644
index 2529f36..0000000
--- a/util/dockerfiles/python3/Dockerfile
+++ /dev/null
@@ -1,37 +0,0 @@
-# Copyright (c) 2020 The Regents of the University of California
-# All Rights Reserved.
-#
-# Redistribution and use in source and binary forms, with or without
-# modification, are permitted provided that the following conditions are
-# met: redistributions of source code must retain the above copyright
-# notice, this list of conditions and the following disclaimer;
-# redistributions in binary form must reproduce the above copyright
-# notice, this list of conditions and the following disclaimer in the
-# documentation and/or other materials provided with the distribution;
-# neither the name of the copyright holders nor the names of its
-# contributors may be used to endorse or promote products derived from
-# this software without specific prior written permission.
-#
-# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
-# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
-# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
-# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
-# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
-# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
-# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
-# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
-# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
-# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
-# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-
-FROM ubuntu:18.04
-
-RUN apt -y update
-RUN apt -y upgrade
-RUN apt -y install build-essential git m4 scons zlib1g zlib1g-dev \
-    libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
-    python-dev python python3-dev python3.6 python-six python3-venv
-
-ENV VIRTUAL_ENV "/venv"
-RUN python3 -m venv $VIRTUAL_ENV
-ENV PATH "$VIRTUAL_ENV/bin:$PATH"