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: Id6a71b7793171ede6de03ec20b5c06a3c7281f5b
Test: presubmit
Bug: 178646865

TEST_MAPPING

 // Generated by cargo2android.py for tests in Android.bp
 {
   "presubmit": [
-    {
-      "host": true,
-      "name": "unicode-width_host_test_src_lib"
-    },
     {
       "name": "unicode-width_device_test_src_lib"
     },
-    {
-      "host": true,
-      "name": "libsqlite3-sys_host_test_src_lib"
-    },
     {
       "name": "libsqlite3-sys_device_test_src_lib"
     }