remove Isotope Mail (unmaintained since 2019) (#3703)

- ref. #3558

README.md

@@ -466,7 +466,6 @@ Simple deployment of [E-mail](https://en.wikipedia.org/wiki/Email) servers, e.g.
 
 - [Cypht](https://cypht.org) - Feed reader for your email accounts. ([Source Code](https://github.com/jasonmunro/cypht)) `LGPL-2.1` `PHP`
 - [IMP](https://www.horde.org/apps/imp/) - HORDE application that provides webmail access to IMAP and POP3 accounts. ([Demo](http://demo.horde.org/), [Source Code](https://www.horde.org/download/imp)) `GPL-2.0` `PHP`
-- [Isotope Mail](https://blog.marcnuri.com/isotope-mail-client-introduction/) - Microservice based webmail client built with ReactJS and Spring. ([Source Code](https://github.com/manusa/isotope-mail)) `Apache-2.0` `Docker/Java`
 - [MailCare](https://mailcare.io) - Open source disposable email address service. ([Source Code](https://github.com/mailcare/mailcare)) `MIT` `PHP`
 - [Roundcube](https://roundcube.net) - Browser-based IMAP client with an application-like user interface. ([Source Code](https://github.com/roundcube/roundcubemail)) `GPL-3.0` `PHP`
 - [SnappyMail](https://snappymail.eu/) - Simple, modern, lightweight & fast web-based email client. (It is an actively developed fork of RainLoop). ([Demo](https://snappymail.eu/demo/), [Source Code](https://github.com/the-djmaze/snappymail)) `AGPL-3.0` `PHP`