In the code base, there are lots of configurations locally guarded by
`stdenv.hostPlatform.is(Darwin|Linux)` despite the targeted options
already being guarded. Examples for these targeted options are:
- `systemd.user.*`: globally guarded by `systemd.user.enable`.
- `launchd.*`: globally guarded by `launchd.enable`.
- `lib.hm.darwin.assertInterval`: only effective on Darwin.
These local guards are an antipattern since they weaken the global
guards. Furthermore, they hamper readability. This series of commits
remove instances of these local guards.
9bfca5b3a7