Clean up rust_test_host TEST_MAPPING after default update am: 6be6a10d6d

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/proc-macro-nested/+/1584272

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Ia0125d4ad0f39ef5aceb69b000e66c1c07ad618f