remove many old version ifdefs

Drop support for building with ghc older than 8.4.4, and with older
versions of serveral haskell libraries than will be included in Debian 10.

The only remaining version ifdefs in the entire code base are now a couple
for aws!

This commit should only be merged after the Debian 10 release.
And perhaps it will need to wait longer than that; it would make
backporting new versions of  git-annex to Debian 9 (stretch) which
has been actively happening as recently as this year.

This commit was sponsored by Ilya Shlyakhter.

Remote/WebDAV.hs

@@ -5,7 +5,6 @@
  - Licensed under the GNU AGPL version 3 or higher.
  -}
 
-{-# LANGUAGE CPP #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 
 module Remote.WebDAV (remote, davCreds, configUrl) where
@@ -16,9 +15,8 @@ import qualified Data.ByteString.Lazy as L
 import qualified Data.ByteString.UTF8 as B8
 import qualified Data.ByteString.Lazy.UTF8 as L8
 import Network.HTTP.Client (HttpException(..), RequestBody)
-#if MIN_VERSION_http_client(0,5,0)
 import qualified Network.HTTP.Client as HTTP
-#endif
+import Network.HTTP.Client (HttpExceptionContent(..), responseStatus)
 import Network.HTTP.Types
 import System.IO.Error
 import Control.Monad.Catch
@@ -42,10 +40,6 @@ import Utility.Url (URLString, matchStatusCodeException, matchHttpExceptionConte
 import Annex.UUID
 import Remote.WebDAV.DavLocation
 
-#if MIN_VERSION_http_client(0,5,0)
-import Network.HTTP.Client (HttpExceptionContent(..), responseStatus)
-#endif
-
 remote :: RemoteType
 remote = RemoteType
 	{ typename = "webdav"
@@ -415,7 +409,6 @@ goDAV (DavHandle ctx user pass _) a = choke $ run $ prettifyExceptions $ do
 {- Catch StatusCodeException and trim it to only the statusMessage part,
  - eliminating a lot of noise, which can include the whole request that
  - failed. The rethrown exception is no longer a StatusCodeException. -}
-#if MIN_VERSION_http_client(0,5,0)
 prettifyExceptions :: DAVT IO a -> DAVT IO a
 prettifyExceptions a = catchJust (matchStatusCodeException (const True)) a go
   where
@@ -428,17 +421,6 @@ prettifyExceptions a = catchJust (matchStatusCodeException (const True)) a go
 		, show (HTTP.path req)
 		]
 	go e = throwM e
-#else
-prettifyExceptions :: DAVT IO a -> DAVT IO a
-prettifyExceptions a = catchJust (matchStatusCodeException (const True)) a go
-  where
-	go (StatusCodeException status _ _) = giveup $ unwords
-		[ "DAV failure:"
-		, show (statusCode status)
-		, show (statusMessage status)
-		]
-	go e = throwM e
-#endif
 
 prepDAV :: DavUser -> DavPass -> DAVT IO ()
 prepDAV user pass = do