Remove old test cert and rest of 4121

Merge from Widevine repo of http://go/wvgerrit/62782

There were still some old test keyboxes in the unit tests for older devices and
in the mod mock.  These are removed.  Also, the cert with system id 7346 has
been removed.

The new test keybox has system ID 7912.
The new test cert has system id 7913.

bug: 76435251
Test: unit tests (unit tests do not pass on v13 oemcrypto)
Test: tested as part of http://go/ag/5501993

Change-Id: I6007b6650162d9dc9d01384faaafc87acdf8ebd7

libwvdrmengine/oemcrypto/test/oec_device_features.h

@@ -14,7 +14,6 @@ class DeviceFeatures {
     NO_METHOD,             // Cannot derive known session keys.
     LOAD_TEST_KEYBOX,      // Call LoadTestKeybox before deriving keys.
     LOAD_TEST_RSA_KEY,     // Call LoadTestRSAKey before deriving keys.
-    EXISTING_TEST_KEYBOX,  // Keybox is already the test keybox.
     FORCE_TEST_KEYBOX,     // User requested calling InstallKeybox.
     TEST_PROVISION_30,     // Device has OEM Certificate installed.
   };