nixos-manual: Further simplify stripAnyPrefixes

First of all this fixes an evaluation error I introduced in ae466ba1

,
which wasn't triggered by any of my own tests against the change because
there are usually no NixOS options that are declared outside of the
<nixpkgs> tree. I renamed the attribute name from "fn" to "fileName"
first and later to "fullPath" but forgot one still occuring "filename".

Thanks to @vcunat for noticing this.

Another thing that he pointed out was that the "stripPrefix" function
can be factored away entirely, because it's very similar to
"removePrefix" in <nixpkgs/lib>.

Unfortunately we can't use "removePrefix" as is, because we need to
account for the final shlash.

So instead of removing it twice and/or retaining "stripPrefix", let's
append a shlash on every "prefixesToStrip" and we can use "removePrefix"
as is.

Tested with:

taalo-build nixos/release.nix -A tests.installer.simple.x86_64-linux

And:

w3m -dump "$(
  nix-build nixos/release.nix -A manual.x86_64-linux
)/share/doc/nixos/options.html"

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