[main] Remove more unused Docker files (#42248)

- clean up the old SystemdActivation files
- follow-up work that's similar to c5c8f438

Co-authored-by: Brennan <brecon@microsoft.com>

src/Servers/Kestrel/test/SystemdActivation/Dockerfile

-FROM microsoft/dotnet-nightly:2.0-runtime-deps
-
-# The "container" environment variable is read by systemd.
-ENV container=docker
-
-# Install and configure dependencies.
-RUN ["apt-get", "-o", "Acquire::Check-Valid-Until=false", "update"]
-RUN ["apt-get", "install", "-y", "--no-install-recommends", "systemd", "socat"]
-
-# Copy .NET installation.
-ADD .dotnet/ /usr/share/dotnet/
-RUN ["ln", "-s", "/usr/share/dotnet/dotnet", "/usr/bin/dotnet"]
-
-# Copy "publish" app.
-ADD publish/ /publish/
-
-# Expose target ports.
-EXPOSE 8080 8081 8082 8083 8084 8085
-
-# Set entrypoint.
-COPY ./docker-entrypoint.sh /
-RUN chmod +x /docker-entrypoint.sh
-ENTRYPOINT ["/docker-entrypoint.sh"]

src/Servers/Kestrel/test/SystemdActivation/docker-entrypoint.sh

-#!/bin/bash
-
-set -e
-
-cd /publish
-systemd-socket-activate -l 8080 -E ASPNETCORE_BASE_PORT=7000 dotnet SystemdTestApp.dll &
-socat TCP-LISTEN:8081,fork TCP-CONNECT:127.0.0.1:7000 &
-socat TCP-LISTEN:8082,fork TCP-CONNECT:127.0.0.1:7001 &
-systemd-socket-activate -l /tmp/activate-kestrel.sock -E ASPNETCORE_BASE_PORT=7100 dotnet SystemdTestApp.dll &
-socat TCP-LISTEN:8083,fork UNIX-CLIENT:/tmp/activate-kestrel.sock &
-socat TCP-LISTEN:8084,fork TCP-CONNECT:127.0.0.1:7100 &
-socat TCP-LISTEN:8085,fork TCP-CONNECT:127.0.0.1:7101 &
-trap 'exit 0' SIGTERM
-wait

src/Servers/Kestrel/test/SystemdActivation/docker.sh

-#!/usr/bin/env bash
-
-set -e
-
-scriptDir=$(dirname "${BASH_SOURCE[0]}")
-dotnetDir="$PWD/.build/.dotnet"
-PATH="$dotnetDir:$PATH"
-dotnet publish -f netcoreapp2.2 ./samples/SystemdTestApp/
-cp -R ./samples/SystemdTestApp/bin/Debug/netcoreapp2.2/publish/ $scriptDir
-cp -R $dotnetDir $scriptDir
-
-image=$(docker build -qf $scriptDir/Dockerfile $scriptDir)
-container=$(docker run -Pd $image)
-
-# Try to connect to SystemdTestApp once a second up to 10 times via all available ports.
-for i in {1..10}; do
-    curl -f http://$(docker port $container 8080/tcp) \
-    && curl -f http://$(docker port $container 8081/tcp) \
-    && curl -fk https://$(docker port $container 8082/tcp) \
-    && curl -f http://$(docker port $container 8083/tcp) \
-    && curl -f http://$(docker port $container 8084/tcp) \
-    && curl -fk https://$(docker port $container 8085/tcp) \
-    && exit 0 || sleep 1;
-done
-
-exit -1