nettle: start maintaining the expression

- consolidate configureFlags
- remove double callPackage; I suspect it's unhealthy for overrides

We haven't needed multiple nettle versions for years I think (d3e488c),
but the split to {default,generic}.nix doesn't seem problematic,
so I kept it to avoid making the history slightly harder to follow.

pkgs/development/libraries/nettle/default.nix

@@ -1,10 +1,10 @@
-{ callPackage, fetchurl, ... } @ args:
+{ callPackage, fetchurl }:
 
-callPackage ./generic.nix (args // rec {
+callPackage ./generic.nix rec {
   version = "3.7.2";
 
   src = fetchurl {
     url = "mirror://gnu/nettle/nettle-${version}.tar.gz";
     sha256 = "0qpi1qp3bcvqdsaxy2pzg530db95x8qjahkynxgwvr6dy5760ald";
   };
-})
+}