X.H.Modal: Remove `Keys` type alias
X.H.Modal: Remove `Keys` type alias

This was only used in three places throughout the module and is sort of
non-standard terminology, hence it might confuse users.  Talking about
functions of type

    XConfig Layout -> M.Map (ButtonMask, KeySym) (X ())

is relatively standard in other modules as well, so it's probably best
to remove the type alias for it.

xmonad/xmonad-contrib,xmonad/xmonad-contrib