Remove "dev" branch reference (for now) (#18242)

README.md

 ## Submitting Changes
 
 - Submit **Bug Fixes** as Pull Requests to the ([bugfix-2.0.x](https://github.com/MarlinFirmware/Marlin/tree/bugfix-2.0.x)) branch.
-- Submit **New Features** to the ([dev-2.1.x](https://github.com/MarlinFirmware/Marlin/tree/dev-2.1.x)) branch.
 - Follow the [Coding Standards](http://marlinfw.org/docs/development/coding_standards.html) to gain points with the maintainers.
 - Please submit your questions and concerns to the [Issue Queue](https://github.com/MarlinFirmware/Marlin/issues).