haskellPackages.libxml-sax: drop obsolete override

Upstream has fixed the implicit declarations in a new release. That
release also includes the test suite in the default package which
unfortunately segfaults, so the build is failing again.

pkgs/development/haskell-modules/configuration-common.nix

@@ -265,9 +265,6 @@ self: super: {
   termbox-bindings-c = appendConfigureFlags [
     "--ghc-option=-optc=-Wno-error=implicit-function-declaration"
   ] super.termbox-bindings-c;
-  libxml-sax = appendConfigureFlags [
-    "--ghc-option=-optc=-Wno-error=implicit-function-declaration"
-  ] super.libxml-sax;
 
   # There are numerical tests on random data, that may fail occasionally
   lapack = dontCheck super.lapack;