nix-manual: drop mdbook-linkcheck dependency

Nix 2.31+ upstream already dropped [output.linkcheck] from
book.toml, so the linkcheck binary is not invoked during the
manual build.

pkgs/tools/package-management/nix/modular/doc/manual/package.nix

@@ -6,7 +6,6 @@
   ninja,
   lowdown-unsandboxed,
   mdbook,
-  mdbook-linkcheck,
   jq,
   python3,
   rsync,
@@ -35,7 +34,6 @@ mkMesonDerivation (finalAttrs: {
     ninja
     (lib.getBin lowdown-unsandboxed)
     mdbook
-    mdbook-linkcheck
     jq
     python3
     rsync