Clean up rust_test_host TEST_MAPPING after default update am: 9a13a8a17e am: 8e178c01e5 am: d6c35b1dc5 am: 8206d010c4

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/syn-mid/+/1584429

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I0254d8daf760936d91a459f559f6a00e86fa4c79