Clean up rust_test_host TEST_MAPPING after default update am: 466121f669 am: baa94cf971 am: c65764e24f am: 0531ec76c3

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/structopt-derive/+/1584431

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I523ffa67ecd7e43f82f46bb8598bffd3eef036fc