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: I9c0fd753736a610ac830f64b83d03c3cde3fbc6d
Test: presubmit
Bug: 178646865
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 3b509e7..99c731f 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -2,22 +2,10 @@
 {
   "presubmit": [
     {
-      "host": true,
-      "name": "futures-core_host_test_src_lib"
-    },
-    {
       "name": "futures-core_device_test_src_lib"
     },
     {
-      "host": true,
-      "name": "futures-util_host_test_src_lib"
-    },
-    {
       "name": "futures-util_device_test_src_lib"
-    },
-    {
-      "host": true,
-      "name": "tokio-macros_host_test_src_lib"
     }
   ]
 }