MM-23800: remove goroutineID and stack printing (#14268)

* MM-23800: remove goroutineID and stack printing

Each hub has a goroutineID which is calculated with a known hack.
The FAQ clearly explains why goroutines don't have an id:
https://golang.org/doc/faq#no_goroutine_id.

We only added that because sometimes the hub would be deadlocked and
having the goroutineID would be useful when getting the stack trace.
This is also problematic in stress tests because the hubs would
frequently get overloaded and the logs would unnecessarily have stack traces.

But that was in the past, and we have done extensive testing with
load tests and fuzz testing to smooth any rough edges remaining.
Including adding additional metrics for hub buffer size.

Monitoring the metrics is a better way to approach this problem.
Therefore, we remove these kludges from the code.

* Also remove deadlock checking code

There is no need for that anymore since
we are getting rid of the stack printing anyways.

Let's do a wholesale refactor and clean up the codebase.

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

app/server.go

@@ -74,9 +74,8 @@ type Server struct {
 	EmailBatching    *EmailBatchingJob
 	EmailRateLimiter *throttled.GCRARateLimiter
 
-	hubsLock                    sync.RWMutex
-	hubs                        []*Hub
-	HubsStopCheckingForDeadlock chan bool
+	hubsLock sync.RWMutex
+	hubs     []*Hub
 
 	PushNotificationsHub PushNotificationsHub