pw_time: Drop board_deps from Zephyr Rust helper

pw_time_backend_zephyr_helper only requires Zephyr headers (@zephyr) to
compile. Depending on @zephyr//:board_deps causes rust_static_library
targets to bundle board support archives into the Rust staticlib,
resulting in duplicate symbol errors during final Zephyr linking.

Change-Id: Iadfe82acd9f549d7a842f7bdcdef875383d62c57
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/490531