This removes the `debugging` bool under GUI configuration, and two no
longer relevant development endpoints: `httpmetrics` (which I can't
imagine anyone using for anything -- if we need such metrics today, the
right place is the Prometheus exported metrics) and the `peerCompletion`
endpoint (previously used by integration tests).

The debugging bool initially enabled just those two endpoints, which are
not for end users. Then we added profiling and support bundles, which
are very useful indeed for end users to access, and they were hidden
behind the same debug flag. I don't see any reason for keeping that flag
now that these methods are more generally useful.

syncthing/docs#949