Clean up rust_test_host TEST_MAPPING after default update am: 0bf8e188a2 am: 6dd5ad0e0b am: 42141167e1

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/downcast-rs/+/1584326

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Ic35f535aeff0b3300199806760591b371239bd0f