consensus: remove CPP from headerRealPoint

GHC 9.4+ "erroneously" requires HasHeader blk. This patch prevents that. I
_think_ the new local signature forces GHC to invoke the HeaderHash (Header
blk) ~ HeaderHash blk rewrite _later_, thereby avoiding that undesirable Wanted
constraint.

Author: nfrisby

ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Block/Abstract.hs

@@ -154,7 +154,7 @@ type instance BlockProtocol (Header blk) = BlockProtocol blk
 
 type instance HeaderHash (Header blk) = HeaderHash blk
 
-instance HasHeader blk => StandardHash (Header blk)
+instance StandardHash blk => StandardHash (Header blk)
 
 -- | Get the 'HeaderFields' of a block, without requiring 'HasHeader blk'
 --

ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Block/RealPoint.hs

@@ -1,4 +1,3 @@
-{-# LANGUAGE CPP #-}
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE OverloadedStrings #-}
@@ -87,17 +86,15 @@ blockRealPoint blk = RealPoint s h
     HeaderFields { headerFieldSlot = s, headerFieldHash = h } = getHeaderFields blk
 
 headerRealPoint ::
-    ( HasHeader (Header blk)
-#if __GLASGOW_HASKELL__ >= 904
--- GHC 9.4+ considers these constraints insufficient.
-    , HasHeader blk
-#endif
-    )
+       forall blk. HasHeader (Header blk)
     => Header blk
     -> RealPoint blk
 headerRealPoint hdr = RealPoint s h
   where
-    HeaderFields { headerFieldSlot = s, headerFieldHash = h } = getHeaderFields hdr
+    HeaderFields { headerFieldSlot = s, headerFieldHash = h } = hf
+
+    hf :: HeaderFields (Header blk)
+    hf = getHeaderFields hdr
 
 realPointToPoint :: RealPoint blk -> Point blk
 realPointToPoint (RealPoint s h) = BlockPoint s h