Revert "Define a class for projecting out the members of a sum."

This reverts commit b7a93aed3d.

semantic-tags/src/Tags/Taggable/Precise.hs

@@ -178,8 +178,3 @@ instance {-# OVERLAPPABLE #-}
       => Element t (l :+: r) where
   prj (R1 r) = prj r
   prj _      = Nothing
-
-
-class GSum t where
-  type Members t :: (* -> *)
-  gmembers :: t a -> Members t a