Use canonical `Monoid` instances

brittany.cabal

@@ -93,7 +93,6 @@ library {
     -Wno-missing-local-signatures
     -Wno-missing-safe-haskell-mode
     -Wno-monomorphism-restriction
-    -Wno-noncanonical-monoid-instances
     -Wno-prepositive-qualified-module
     -Wno-redundant-constraints
     -Wno-unsafe

src/Language/Haskell/Brittany/Internal/Config/Types.hs

@@ -264,22 +264,16 @@ instance Semigroup.Semigroup (CConfig Identity) where
 
 instance Monoid (CDebugConfig Maybe) where
   mempty = gmempty
-  mappend = gmappend
 instance Monoid (CLayoutConfig Maybe) where
   mempty = gmempty
-  mappend = gmappend
 instance Monoid (CErrorHandlingConfig Maybe) where
   mempty = gmempty
-  mappend = gmappend
 instance Monoid (CForwardOptions Maybe) where
   mempty = gmempty
-  mappend = gmappend
 instance Monoid (CPreProcessorConfig Maybe) where
   mempty = gmempty
-  mappend = gmappend
 instance Monoid (CConfig Maybe) where
   mempty = gmempty
-  mappend = gmappend
 
 
 data IndentPolicy = IndentPolicyLeft -- never create a new indentation at more