Clean up rust_test_host TEST_MAPPING after default update am: f706f8b82f

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/proc-macro-error/+/1590595

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Ie549af2f9e793a3ec5c8546d27375f1f41be047e
diff --git a/TEST_MAPPING b/TEST_MAPPING
deleted file mode 100644
index 1e756de..0000000
--- a/TEST_MAPPING
+++ /dev/null
@@ -1,9 +0,0 @@
-// Generated by cargo2android.py for tests in Android.bp
-{
-  "presubmit": [
-    {
-      "host": true,
-      "name": "structopt-derive_host_test_src_lib"
-    }
-  ]
-}