nixos-render-docs: drop frozendict

with mypy type checking and Mapping types this is a lot less useful than
anticipated. let's drop it for simplicity and having fewer dependencies.

frozendict 2.3.5 also broke the mypy checks.

pkgs/tools/nix/nixos-render-docs/src/nixos_render_docs/docbook.py

@@ -1,5 +1,4 @@
 from collections.abc import Mapping, MutableMapping, Sequence
-from frozendict import frozendict # type: ignore[attr-defined]
 from typing import Any, cast, Optional, NamedTuple
 
 import markdown_it