nixos/polkit: remove root from adminIdentities

Fixes https://github.com/NixOS/nixpkgs/issues/75075.

To summarize the report in the aforementioned issue, at a glance,
it's a different default than what upstream polkit has. Apparently
for 8+ years polkit defaults admin identities as members of
the wheel group [0]. This assumption would be appropriate on NixOS, where
every member of group 'wheel' is necessarily privileged.

[0]: https://gitlab.freedesktop.org/polkit/polkit/commit/763faf434b445c20ae9529100d3ef5290976d0c9