PLT-5900 Removed automatic configuration of SiteURL (#5952)

* Revert "PLT-5765 Passed SiteURL to SendNotifications (#5705)"

This reverts commit a8e68bd890.

* Removed automatic configuration of SiteURL

* Reverted unintentional config changes

* Updated help text

* Added link to docs in Site URL warning

* Fixed merge conflict

app/command_join.go

@@ -45,7 +45,7 @@ func (me *JoinProvider) DoCommand(args *model.CommandArgs, message string) *mode
 				return &model.CommandResponse{Text: args.T("api.command_join.fail.app_error"), ResponseType: model.COMMAND_RESPONSE_TYPE_EPHEMERAL}
 			}
 
-			if err := JoinChannel(channel, args.UserId, args.SiteURL); err != nil {
+			if err := JoinChannel(channel, args.UserId); err != nil {
 				return &model.CommandResponse{Text: args.T("api.command_join.fail.app_error"), ResponseType: model.COMMAND_RESPONSE_TYPE_EPHEMERAL}
 			}