Delete reference to cohorts repo.

Cohorts repo was closed after an RFC.
Resolves #780

Author: waciumawanjohi

README.md

@@ -62,7 +62,6 @@ Decide how much or how little to spend based on your own time and budget;
 just remember that you can't purchase success!
 
 **Process**. Students can work through the curriculum alone or in groups, in order or out of order.
-- For grouping up, please use the [cohorts repository](https://github.com/ossu/cohorts) to find or create a cohort suited to you.
 - We recommend doing all courses in Core CS, only skipping a course when you are certain that you've already learned the material previously.
 - For simplicity, we recommend working through courses (especially Core CS) in order from top to bottom, as they have already been [topologically sorted](https://en.wikipedia.org/wiki/Topological_sorting) by their prerequisites.
 - Courses in Advanced CS are electives. Choose one subject (e.g. Advanced programming) you want to become an expert in and take all the courses under that heading. You can also create your own custom subject, but we recommend getting validation from the community on the subject you choose.