[Xmonad] minor code comment

[Donald Bruce Stewart]

mvanier:
> In StackSet.hs, the "Eq a" qualification in
> 
> index :: Eq a => StackSet i a s sd -> [a]
> index = with [] integrate
> 
> is superfluous.  Is there any good reason to keep it?

Not unless it is required (its harmless dead code though, the Eq
dictionary will be removed).

-- Don