Clean up rust_test_host TEST_MAPPING after default update am: cadc338164 am: 377ba206c2

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/smallvec/+/1590018

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: If0375fe66d084ae81c254cca2459339b326b7e3f