Clean up rust_test_host TEST_MAPPING after default update am: 812115654b

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/unicode-segmentation/+/1582857

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Iacdcdc2b6591403462a0bf0d23534a13b60612ca