Clean up rust_test_host TEST_MAPPING after default update

After b/177689340, rust_test_host doesn't need TEST_MAPPING config anymore to run in
presubmit.

Change-Id: I30d7d66ab1e291f0abbf5b7211df5cf5690d5f96
Test: presubmit
Bug: 178646865
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"
-    }
-  ]
-}