Fix building against Lens 5 by commenting out type signatures(!).

Taken from upstream:

  https://github.com/ganeti/ganeti/commit/5e30bad1bba63c9f6c782003ef2560f107a0ba24

diff --git a/src/Ganeti/Network.hs b/src/Ganeti/Network.hs
index 1cb6aa1ec..696c1cd1b 100644
--- a/src/Ganeti/Network.hs
+++ b/src/Ganeti/Network.hs
@@ -87,11 +87,11 @@ data PoolPart = PoolInstances | PoolExt
 addressPoolIso :: Iso' AddressPool BA.BitArray
 addressPoolIso = iso apReservations AddressPool
 
-poolLens :: PoolPart -> Lens' Network (Maybe AddressPool)
+--poolLens :: PoolPart -> Lens' Network (Maybe AddressPool)
 poolLens PoolInstances = networkReservationsL
 poolLens PoolExt = networkExtReservationsL
 
-poolArrayLens :: PoolPart -> Lens' Network (Maybe BA.BitArray)
+--poolArrayLens :: PoolPart -> Lens' Network (Maybe BA.BitArray)
 poolArrayLens part = poolLens part . mapping addressPoolIso
 
 netIpv4NumHosts :: Network -> Integer
diff --git a/src/Ganeti/Utils/MultiMap.hs b/src/Ganeti/Utils/MultiMap.hs
index d54da3ab0..279e9335a 100644
--- a/src/Ganeti/Utils/MultiMap.hs
+++ b/src/Ganeti/Utils/MultiMap.hs
@@ -91,7 +91,7 @@ multiMap :: (Ord k, Ord v) => M.Map k (S.Set v) -> MultiMap k v
 multiMap = MultiMap . M.filter (not . S.null)
 
 -- | A 'Lens' that allows to access a set under a given key in a multi-map.
-multiMapL :: (Ord k, Ord v) => k -> Lens' (MultiMap k v) (S.Set v)
+--multiMapL :: (Ord k, Ord v) => k -> Lens' (MultiMap k v) (S.Set v)
 multiMapL k f = fmap MultiMap
                  . at k (fmap (mfilter (not . S.null) . Just)
                          . f . fromMaybe S.empty)
-- 
2.41.0