Remove unused imports (with newer protolude)

The changes here are from fixing stack build with LTS 15.8 (GHC 8.8.3)
to be warning-free. Most if not all come from the newer protolude
version, which adds some things like `unwords`.

So to make sure that we don't get unused import warnings with newer
stackage, this both raises the protolude lower bound and removes
these imports.

src/PostgREST/App.hs

@@ -40,7 +40,6 @@ import Network.HTTP.Types.Status
 import Network.Wai
 
 import PostgREST.ApiRequest       (Action (..), ApiRequest (..),
-                                   ContentType (..),
                                    InvokeMethod (..), Target (..),
                                    mutuallyAgreeable, userApiRequest)
 import PostgREST.Auth             (containsRole, jwtClaims,