misc: Removed python3 Dockerfile

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>