move platinum tests from staging to live

no observed flakiness after 2+ weeks in staging bucket.

Bug: 140253805
Test: atest :platinum-postsubmit
Change-Id: I5c9149188d996b54eded2c14458a9b5513057bf8
diff --git a/media/TEST_MAPPING b/media/TEST_MAPPING
index 206f87f..b006f38 100644
--- a/media/TEST_MAPPING
+++ b/media/TEST_MAPPING
@@ -48,7 +48,7 @@
         }
     ],
 
-    "staged-platinum-postsubmit": [
+    "platinum-postsubmit": [
         // runs regularly, independent of changes in this tree.
         // signals if changes elsewhere break media functionality
         {