Don't turn Prometheus on in --dev mode (#43129)

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

- [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/guides/committing-changes.md#changes-files)
for more information.

## Testing

- [ ] QA'd all new/changed functionality manually

changes/turn-off-prom-in-dev-mode

@@ -0,0 +1 @@
+* No longer turn on Prometheus serving by default with a hard-coded username and password when the server is started with `--dev`.

cmd/fleet/main.go

@@ -108,9 +108,6 @@ func applyDevFlags(cfg *config.FleetConfig) {
 	// that match our default dev setup.
 	setIfEmpty(&cfg.Mysql.Password, "insecure")
 
-	setIfEmpty(&cfg.Prometheus.BasicAuth.Username, "fleet")
-	setIfEmpty(&cfg.Prometheus.BasicAuth.Password, "insecure")
-
 	setIfEmpty(&cfg.S3.CarvesBucket, "carves-dev")
 	setIfEmpty(&cfg.S3.CarvesRegion, "localhost")
 	setIfEmpty(&cfg.S3.CarvesPrefix, "dev-prefix")