nixos/doc: Revert allowing olinks from options

This reverts commit 1d77dcae

.

It will be reintroduced along with #14700 as a separate branch, as
suggested by @nbp.

I added this to this branch because I thought it was a necessary
dependency, but it turns out that the build of the manual/manpages still
succeeds and merely prints a warning like this:

warning: failed to load external entity "olinkdb.xml"
Olink error: could not open target database 'olinkdb.xml'.
Error: unresolved olink: targetdoc/targetptr = 'manual/module-taskserver'.

The olink itself will be replaced by "???", so users looking at the
description of the option in question will still see the reference to
the NixOS manual, like this:

More instructions about NixOS in conjuction with Taskserver can be found
in the NixOS manual at ???.

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