shellcheck is packaged twice

@peti 

In this commit I removed `development/libraries/haskell/ShellCheck` and renamed `haskellPackages.ShellCheck` (in `development/tools/misc/ShellCheck`) to `haskellPackages.shellCheck` (because lower case is more common).

I know that I should not edit these files manually, but at least, the information is there now. I guess that someone can fix the upstream stuff.
The spelling of the attribute name should match the package name, i.e. `ShellCheck` was correct.