MM-61225: Revert session pooling (#28901)

This originated from https://github.com/mattermost/mattermost/issues/15249.

However, the original idea was discarded https://github.com/mattermost/mattermost/issues/15249#issuecomment-709713065
as being too complicated to implement. Then I had another
idea to implement it just for session objects.

My thinking was that since every single request allocates a new
session struct, it would be good to use a sync.Pool for that.

However, 4 years later, now we know that the primary bottleneck
in app performance comes from websocket event marshalling.
Therefore, while it would be good to do this, it is difficult
to do it correctly (as shown by the numerous racy tests).

Hence, reverting this.

```release-note
NONE
```

server/channels/app/platform/session.go

@@ -14,19 +14,6 @@ import (
 	"github.com/mattermost/mattermost/server/v8/platform/services/cache"
 )
 
-func (ps *PlatformService) ReturnSessionToPool(session *model.Session) {
-	if session != nil {
-		session.Id = ""
-		// All existing prop fields are cleared once the session is retrieved from the pool.
-		// To speed up that process, clear the props here to avoid doing that in the hot path.
-		//
-		// If the request handler spawns a goroutine that uses the session, it might race with this code.
-		// In that case, the handler should copy the session and use the copy in the goroutine.
-		clear(session.Props)
-		ps.sessionPool.Put(session)
-	}
-}
-
 func (ps *PlatformService) CreateSession(c request.CTX, session *model.Session) (*model.Session, error) {
 	session.Token = ""
 
@@ -133,8 +120,8 @@ func (ps *PlatformService) ClearAllUsersSessionCache() {
 }
 
 func (ps *PlatformService) GetSession(c request.CTX, token string) (*model.Session, error) {
-	var session = ps.sessionPool.Get().(*model.Session)
-	if err := ps.sessionCache.Get(token, session); err == nil {
+	var session model.Session
+	if err := ps.sessionCache.Get(token, &session); err == nil {
 		if m := ps.metricsIFace; m != nil {
 			m.IncrementMemCacheHitCounterSession()
 		}
@@ -145,7 +132,7 @@ func (ps *PlatformService) GetSession(c request.CTX, token string) (*model.Sessi
 	}
 
 	if session.Id != "" {
-		return session, nil
+		return &session, nil
 	}
 
 	return ps.GetSessionContext(c, token)
@@ -206,8 +193,6 @@ func (ps *PlatformService) RevokeSession(c request.CTX, session *model.Session)
 func (ps *PlatformService) RevokeAccessToken(c request.CTX, token string) error {
 	session, _ := ps.GetSession(c, token)
 
-	defer ps.ReturnSessionToPool(session)
-
 	schan := make(chan error, 1)
 	go func() {
 		schan <- ps.Store.Session().Remove(token)