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/Error.hs

@@ -23,7 +23,6 @@ import qualified Network.HTTP.Types.Status as HT
 
 import Data.Aeson  ((.=))
 import Network.Wai (Response, responseLBS)
-import Text.Read   (readMaybe)
 
 import Network.HTTP.Types.Header