nixos/make-options-doc: Remove now redundant fixup

This fixes a problem where most NixOS option declaration paths started
with `nixos/modules/nixos/modules/`, which should have been only
just `nixos/modules/`.

Since dcc0ee9ea1, we use a source path
that includes both the `modules` and `nixos/modules`, so we don't need
to additionally prefix `nixos/modules` anymore.

Manually checked with

    nix-build nixos/release.nix -A options
    jq < result/share/doc/nixos/options.json '[ ."_module.args".declarations, ."meta.maintainers".declarations, ."nixpkgs.system".declarations, ."zramSwap.swapDevices".declarations ]'

Samples:

- `_module` for options defined in `lib/`
- `meta.maintainers` for options defined in `modules/`
- `nixpkgs.system` for options defined in `nixos/` with `meta.buildDocsInSandbox = false;`
- `zramSwap.swapDevices` for regular `nixos/` without that

I'm a big fan of automated tests, but in this case I am not doing that because
- the solution is a simplification, and
- no obvious place for the test, and it would couple with unrelated
  downstream code more than I would like.

nixos/lib/make-options-doc/mergeJSON.py

@@ -52,12 +52,6 @@ for arg in sys.argv[1:]:
 options = pivot(json.load(open(sys.argv[1 + optOffset], 'r')))
 overrides = pivot(json.load(open(sys.argv[2 + optOffset], 'r')))
 
-# fix up declaration paths in lazy options, since we don't eval them from a full nixpkgs dir
-for (k, v) in options.items():
-    # The _module options are not declared in nixos/modules
-    if v.value['loc'][0] != "_module":
-        v.value['declarations'] = list(map(lambda s: f'nixos/modules/{s}' if isinstance(s, str) else s, v.value['declarations']))
-
 # merge both descriptions
 for (k, v) in overrides.items():
     cur = options.setdefault(k, v).value