[MM-47492] - Don't count system messages as replies to a thread (#23022)

* [MM-47492] - Don't count system messages as replies to a thread

* Add test assertion

* revert store changes`

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Co-authored-by: Nevyana Angelova <nevyangelova@Nevyanas-MacBook-Pro.local>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/channel.go

@@ -2408,7 +2408,6 @@ func (a *App) PostAddToChannelMessage(c request.CTX, user *model.User, addedUser
 		Message:   message,
 		Type:      postType,
 		UserId:    user.Id,
-		RootId:    postRootId,
 		Props: model.StringInterface{
 			"userId":                   user.Id,
 			"username":                 user.Username,