Remove reliable websockets configuration (#19040)

The feature is stable enough to be removed
as a config knob.

For backwards compatiblity with older mobile
clients, we keep sending the config param
as true for client config requests.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4/7m95frqbk9o8zxin6xo9c7iusoh

```release-note
NONE
```

model/config.go

@@ -365,7 +365,6 @@ type ServiceSettings struct {
 	ThreadAutoFollow                                  *bool   `access:"experimental_features"`
 	CollapsedThreads                                  *string `access:"experimental_features"`
 	ManagedResourcePaths                              *string `access:"environment_web_server,write_restrictable,cloud_restrictable"`
-	EnableReliableWebSockets                          *bool   `access:"experimental_features"` // telemetry: none
 }
 
 func (s *ServiceSettings) SetDefaults(isUpdate bool) {
@@ -782,10 +781,6 @@ func (s *ServiceSettings) SetDefaults(isUpdate bool) {
 	if s.ManagedResourcePaths == nil {
 		s.ManagedResourcePaths = NewString("")
 	}
-
-	if s.EnableReliableWebSockets == nil {
-		s.EnableReliableWebSockets = NewBool(true)
-	}
 }
 
 type ClusterSettings struct {