Use floskell from hackage

[lukel97]

Thanks to @ennocramer for the upload ennocramer/floskell#6 (comment)

[alanz]

@bubba did you follow the process outlined in the top of .gitmodules to remove the submodule?

[lukel97]

@alanz I've deleted and checked in submodules/floskell folder, but was not able to find/delete it in .git/modules, although this is inside a different worktree, whilst I also have master checked at the same time.