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: Ib34e1a2a14a9e00b2fd48fd829cce2f4f79d7f65
Test: presubmit
Bug: 178646865
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 824dcad..30d2306 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -2,15 +2,7 @@
 {
   "presubmit": [
     {
-      "host": true,
-      "name": "fnv_host_test_lib"
-    },
-    {
       "name": "fnv_device_test_lib"
-    },
-    {
-      "host": true,
-      "name": "tokio-macros_host_test_src_lib"
     }
   ]
 }