Clean up rust_test_host TEST_MAPPING after default update am: 0bf8e188a2 am: 6dd5ad0e0b am: 42141167e1 am: 725d06222a

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

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I2bbb34edc279119d92a985b904841e91ca4321b6