Clean up rust_test_host TEST_MAPPING after default update am: b29d50ec96 am: 9f5d956854 am: df7743ea99 am: f21617a5ba

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/num_cpus/+/1591331

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I568bcbf296f0c51d196b2b5bcf0c1be429329e29