fix: remove code that was setting mail_url to 'undefined' (#30055)

.changeset/pink-zoos-join.md

+---
+"@rocket.chat/meteor": patch
+---
+
+Fix the code that was setting email URL to an invalid value when SMTP was not set

apps/meteor/app/lib/server/startup/settingsOnLoadSMTP.ts

@@ -5,7 +5,7 @@ settings.watchMultiple(['SMTP_Host', 'SMTP_Port', 'SMTP_Username', 'SMTP_Passwor
 	SystemLogger.info('Updating process.env.MAIL_URL');
 
 	if (!settings.get('SMTP_Host')) {
-		process.env.MAIL_URL = undefined;
+		delete process.env.MAIL_URL;
 		return;
 	}