Clean up rust_test_host TEST_MAPPING after default update am: 812115654b am: db69b218e5 am: 8a8335ecec

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

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I864f9be1037bc556893db2d6c41f4b1758c16aee