Remove OpenNote. (#3782)

Remove OpenNote because it's discontinued.

README.md

@@ -1430,7 +1430,6 @@ _Related: [Wikis](#wikis)_
 - [MiniNote](https://github.com/muety/mininote) - Simple Markdown note-taking app with persistence. `MIT` `Nodejs`
 - [Notes'n'Todos](https://github.com/larspontoppidan/notesntodos) - Write notes and todos online in markdown with tag filtering and date sorting. ([Demo](https://lpss.dk/nnt-playground/)) `MIT` `Python`
 - [Oddmuse](https://oddmuse.org/) - A simple wiki engine written in Perl. No database required. ([Source Code](https://github.com/kensanata/oddmuse)) `GPL-3.0` `Perl`
-- [OpenNote](https://github.com/FoxUSA/OpenNote) - OpenNote was built to be an open web-based alternative to Microsoft OneNote (T) and EverNote. ([Demo](https://foxusa.github.io/OpenNote/OpenNote/#/folder)) `MIT` `HTML5`
 - [Overleaf](https://www.overleaf.com/) - Web-based collaborative LaTeX editor. ([Source Code](https://github.com/overleaf/overleaf)) `AGPL-3.0` `Ruby`
 - [Plainpad](https://alextselegidis.com/get/plainpad/) - A modern note taking application for the cloud, utilizing the best features of progressive web apps technology. ([Demo](https://alextselegidis.com/try/plainpad/), [Source Code](https://github.com/alextselegidis/plainpad)) `GPL-3.0` `PHP`
 - [savepad](https://github.com/shelltr/textpad) - Minimalist notepad based on notepad.cc. `MIT` `PHP`