Remove Unnecessary Locking from Android

(This is a merge of http://go/wvgerrit/14242)

To try to fix b/17761616, I added a lock to the DRM Plugin. However,
the API guarantees that any single DrmPlugin will never be called into
from multiple threads at the same time, rendering this unnecessary.
Previously, b/18194977 meant that this was not always true, but now
that that bug is fixed, this lock is not needed and can be removed.

Bug: 18194977
Change-Id: I25ee5fa44f35706f863b036997453441eac29c95

libwvdrmengine/mediadrm/include/WVDrmPlugin.h

@@ -16,7 +16,6 @@
 #include "utils/Errors.h"
 #include "utils/KeyedVector.h"
 #include "utils/List.h"
-#include "utils/Mutex.h"
 #include "utils/String8.h"
 #include "utils/Vector.h"
 #include "wv_cdm_event_listener.h"
@@ -27,7 +26,6 @@ namespace wvdrm {
 
 using android::KeyedVector;
 using android::List;
-using android::Mutex;
 using android::status_t;
 using android::String8;
 using android::Vector;
@@ -253,8 +251,6 @@ class WVDrmPlugin : public android::DrmPlugin,
   WvContentDecryptionModule* mCDM;
   WVGenericCryptoInterface* mCrypto;
   std::string mOrigin;
-
-  Mutex mCryptoSessionsMutex;
   map<CdmSessionId, CryptoSession> mCryptoSessions;
 
   status_t queryProperty(const std::string& property,