fix(tracing): remove unused imports

[skot]

Summary

• make Linux-only tracing imports conditional
• remove the unused local tracing prelude import
• keep the journald fallback log message via a fully qualified macro

Validation

• cargo test --no-run

[skot]

this PR is to support building mujina on macos without warnings.

[skot]

there is some overlap here with #32

[rkuester]

Thanks for filing. I took the issues raised here, along with the overlap in #32, as a hint that conditional compilation of the journald features required a bit of a refactoring to consolidate the #[cfg]s multiplying throughout the file. See 3176b56.