Remove fleetctl binary from fleetdm/fleet image and remove unused Dockerfile (#25749)

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
```

- [X] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/Committing-Changes.md#changes-files)
for more information.
- [x] Manual QA for all new/changed functionality

tools/fleet-docker/Dockerfile

@@ -10,6 +10,5 @@ RUN addgroup -S fleet && adduser -S fleet -G fleet
 USER fleet
 
 COPY fleet /usr/bin/
-COPY fleetctl /usr/bin/
 
 CMD ["fleet", "serve"]