lix: drop mdbook-linkcheck dependency

Lix 2.93/2.94 still reference [output.linkcheck] in book.toml,
but mdbook-linkcheck2 installs as binary 'mdbook-linkcheck2'
which mdbook cannot discover via [output.linkcheck]. Substituting
mdbook-linkcheck2 would not work due to this binary name mismatch.

Dropping it entirely for now seems safe, lix upstream commented it out
in 54df89f601b3. And the build still passes.

pkgs/tools/package-management/lix/common-lix.nix

@@ -50,7 +50,6 @@ assert lib.assertMsg (
   lsof,
   mercurial,
   mdbook,
-  mdbook-linkcheck,
   nlohmann_json,
   ninja,
   openssl,
@@ -179,7 +178,6 @@ stdenv.mkDerivation (finalAttrs: {
   ++ lib.optionals enableDocumentation [
     (lib.getBin lowdown-unsandboxed)
     mdbook
-    mdbook-linkcheck
     doxygen
   ]
   ++ lib.optionals (hasDtraceSupport && withDtrace) [ systemtap-sdt ]