Revert "Semigroup instance for [a]"

This reverts commit 1b35383.

Author: paf31

src/Data/Array.purs

@@ -180,9 +180,6 @@ instance eqArray :: (Eq a) => Eq [a] where
   (==) _ _ = false
   (/=) xs ys = not (xs == ys)
 
-instance semigroupArray :: Semigroup [a] where
-  (<>) = concat
-
 instance monadArray :: Monad [] where
   return = singleton
   (>>=) = flip concatMap