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

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

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I333fe7ddc351baab467a8bd24258aa5423421d14