boost: drop unused expat dependency

The `expat` was removed upstream in 10135eeab5f7cd28e43f00699dea724231cd7556 [1].
Boost used SVN at the time making precise timeline checks bothersome,
but it seems the first release including this drop was in late 2010.

Nixpkgs had the expat include logic in 00b296762a for version 1.43.
We no longer maintain such ancient versions of boost, yet the logic was still there.

[1] https://github.com/boostorg/graph/commit/10135eeab5f7cd28e43f00699dea724231cd7556

pkgs/development/libraries/boost/generic.nix

@@ -2,7 +2,6 @@
   lib,
   stdenv,
   icu,
-  expat,
   zlib,
   bzip2,
   zstd,
@@ -87,8 +86,6 @@ let
       "variant=${variant}"
       "threading=${threading}"
       "link=${link}"
-      "-sEXPAT_INCLUDE=${expat.dev}/include"
-      "-sEXPAT_LIBPATH=${expat.out}/lib"
     ]
     ++ lib.optionals (lib.versionAtLeast version "1.85") [
       (
@@ -340,7 +337,6 @@ stdenv.mkDerivation {
   ] ++ lib.optional stdenv.hostPlatform.isDarwin fixDarwinDylibNames;
   buildInputs =
     [
-      expat
       zlib
       bzip2
       libiconv