Remove Unused OEMCrypto Wrapper Methods

(This is a merge of
https://widevine-internal-review.googlesource.com/#/c/10141/
from the Widevine CDM repo.)

Over the course of revising the CAST support, the need for the
MediaDrm to call open and close session was removed. However, the
methods to do that were not. This removes the unused code.

Change-Id: I577f5314b6b20f36c308ac6753f1084ff3c707d9

libwvdrmengine/mediadrm/test/WVDrmPlugin_test.cpp

@@ -96,10 +96,6 @@ class MockCrypto : public WVGenericCryptoInterface {
                                        size_t, OEMCrypto_Algorithm,
                                        const uint8_t*, size_t));
 
-  MOCK_METHOD1(openSession, OEMCryptoResult(OEMCrypto_SESSION*));
-
-  MOCK_METHOD1(closeSession, OEMCryptoResult(OEMCrypto_SESSION));
-
   MOCK_METHOD3(loadDeviceRSAKey, OEMCryptoResult(OEMCrypto_SESSION,
                                                  const uint8_t*, size_t));