Revert "haxe: Use ignoreExternals and fix build."

This reverts commit a0716f28

.

Starting with r6706, externals now have a fixed revision, so we no longer need
to ignore them.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

Conflicts:
	pkgs/development/compilers/haxe/default.nix