Remove fleetctl binary from fleetdm/fleet image and remove unused Dockerfile

[lucasmrod]

For #25748.

Manually tested by:

1. Building a fleet executable for Linux on a Ubuntu VM (with -extldflags "-static") for Alpine.
2. Placing the executable in tools/fleet-docker.
3. Building a local docker image using docker build -t fleetdm/fleet:v42.42.42 --platform=linux/amd64 ..
4. Running the docker image (using docker run) and use Fleet on the browser.

docker run -v $(pwd)/../osquery:/run -p 8412:8412 -e FLEET_MYSQL_ADDRESS=host.docker.internal:3306 -e FLEET_MYSQL_DATABASE=fleet -e FLEET_MYSQL_USERNAME=fleet -e FLEET_MYSQL_PASSWORD=insecure -e FLEET_REDIS_ADDRESS=host.docker.internal:6379 -e FLEET_SERVER_ADDRESS=0.0.0.0:8412 -e FLEET_SERVER_CERT=/run/fleet.crt -e FLEET_SERVER_KEY=/run/fleet.key -e FLEET_LOGGING_JSON='true' -e FLEET_VULNERABILITIES_CURRENT_INSTANCE_CHECKS="yes" -e FLEET_VULNERABILITIES_DATABASES_PATH=/vulndb -e FLEET_LOGGING_DEBUG='true' -it fleetdm/fleet:v42.42.42

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
  See Changes files for more information.
• Manual QA for all new/changed functionality

[lucasmrod]

Seems our release scripts use tools/fleet-docker/Dockerfile (which almost looks the same).

[lucasmrod]

Haven't found any references to these Makefile targets.

[lucasmrod]

Let me know if there's a reason to keep the fleetctl executable on the fleetdm/fleet docker image.