control/controlclient,health,ipn/ipnlocal,health: fix deadlock by deleting health reporting

A recent change (009d702adf) introduced a deadlock where the
/machine/update-health network request to report the client's health
status update to the control plane was moved to being synchronous
within the eventbus's pump machinery.

I started to instead make the health reporting be async, but then we
realized in the three years since we added that, it's barely been used
and doesn't pay for itself, for how many HTTP requests it makes.

Instead, delete it all and replace it with a c2n handler, which
provides much more helpful information.

Fixes tailscale/corp#32952

Change-Id: I9e8a5458269ebfdda1c752d7bbb8af2780d71b04
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

health/state.go

@@ -14,6 +14,9 @@ import (
 
 // State contains the health status of the backend, and is
 // provided to the client UI via LocalAPI through ipn.Notify.
+//
+// It is also exposed via c2n for debugging purposes, so try
+// not to change its structure too gratuitously.
 type State struct {
 	// Each key-value pair in Warnings represents a Warnable that is currently
 	// unhealthy. If a Warnable is healthy, it will not be present in this map.