Removing Mjolnir docsets

New way to get these docs (per http://mjolnir.io/) is to use our own hosted docset feed:

~~~sh
$ open 'dash-feed://https%3A%2F%2Fraw.githubusercontent.com%2Fmjolnir-io%2Fmjolnir-docs%2Fmaster%2FMjolnir.xml'
~~~

Thanks @Kapeli for all your help :)