Remove unvaluable exercises

[Dedekind561]

Changes

Part of review from here CodeYourFuture/syllabus#499

• Removes old exercises dir - exercises in there are really trivial and the syllabus lesson exercises already assess those basic skills anyway so seems pointless to do it twice.
• Also removes reference to + operator in strings to encourage trainees to use template literals from the get-go
• Updates language wrt TDD. We're currently not using TDD to check their work. We're using tests that were written outside the TDD red-green-refactor cycle. It doesn't make sense to refer to TDD here.

[SallyMcGrath]

LGTM

This following comment is not blocking for these changes.

I agree these exercises are not good value. The practice is good for some trainees, however, so how about linking them to some exercise lists as a resource in the coursework instead?

Possibilities:

https://exercism.org/tracks/javascript/exercises
https://www.jschallenger.com/javascript-basics
https://www.executeprogram.com/courses/javascript-array/lessons/basics

[SallyMcGrath]

LGTM 🎉