Clean up rust_test_host TEST_MAPPING after default update am: d4bc4a1f1e

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/futures-util/+/1582281

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I89f8eca777ccdea8a52daed159471c0eaa6ede44