Remove platform cert from Tethering tests

Bug: 156866746
Test: atest TetheringTests, TetheringCoverageTests
Change-Id: I7c539f1f4a447b5913164b222601c6113c6fe645

Tethering/tests/integration/Android.bp

@@ -63,7 +63,6 @@ android_test {
 // NetworkStackTests.
 android_test {
     name: "TetheringCoverageTests",
-    certificate: "platform",
     platform_apis: true,
     test_suites: ["device-tests", "mts"],
     test_config: "AndroidTest_Coverage.xml",

Tethering/tests/unit/Android.bp

@@ -82,7 +82,7 @@ android_library {
 
 android_test {
     name: "TetheringTests",
-    certificate: "platform",
+    platform_apis: true,
     test_suites: [
         "device-tests",
         "mts",