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: Ia1324168c0028c301b06610ab8cd7f0ac669356c
Test: presubmit
Bug: 178646865
diff --git a/TEST_MAPPING b/TEST_MAPPING
index b0d11cf..7795040 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -2,17 +2,9 @@
 {
   "presubmit": [
     {
-      "host": true,
-      "name": "futures-util_host_test_src_lib"
-    },
-    {
       "name": "futures-util_device_test_src_lib"
     },
     {
-      "host": true,
-      "name": "futures-task_host_test_src_lib"
-    },
-    {
       "name": "futures-task_device_test_src_lib"
     }
   ]