parallel-letter-frequency: Mark exercise as foregone

[N-Parsons]

Resolves #1106 by marking parallel-letter-frequency as foregone and removing the exercise directory.

I'm open to discussion on whether the directory and (incorrect) implementation should remain in the repo, but I have removed it because the current implementation is wrong (and not parallel) and so I feel that it holds no value to retain it.

[N-Parsons]

I'm going to go ahead and merge this now, since there are issues with this exercise that could put learners off.

[cg-cnu]

Thanks @N-Parsons for removing it. Just working on it and the title and instructions are not clear and seems to be a bottleneck for many people to move forward.
I guess the numbers says it all

[N-Parsons]

@cg-cnu, it was a fairly recent addition to the track (9 days ago), which is most of the reason why the numbers are so low. Hopefully people weren't too put off during that time - we'll be more careful in future with where in the config each exercise is being placed, although the upcoming revision to the way Exercism works should also stop this kind of thing from happening.

[cg-cnu]

Hey @N-Parsons Sorry, its a misunderstanding. Didn't know that its a recent addition. You guys are doing awesome work ❤️ Thanks for your time 🙇