reverseproxy: api: Remove misleading 'healthy' value

In v2.5.0, upstream health was fixed such that whether an upstream is
considered healthy or not is mostly up to each individual handler's
config. Since "healthy" is an opinion, it is not a global value.

I unintentionally left in the "healthy" field in the API endpoint for
checking upstreams, and it is now misleading (see #4792).

However, num_requests and fails remains, so health can be determined by
the API client, rather than having it be opaquely (and unhelpfully)
determined for the client.

If we do restore this value later on, it'd need to be replicated once
per reverse_proxy handler according to their individual configs.

modules/caddyhttp/reverseproxy/admin.go

@@ -35,7 +35,6 @@ type adminUpstreams struct{}
 // upstreamResults holds the status of a particular upstream
 type upstreamStatus struct {
 	Address     string `json:"address"`
-	Healthy     bool   `json:"healthy"`
 	NumRequests int    `json:"num_requests"`
 	Fails       int    `json:"fails"`
 }