Delete unused mutex in HWC2to1Adapter

Change-Id: I2fe5b8b6d35c07fc2243c88d5739a25be5c61686
diff --git a/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.h b/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.h
index daa988c..fee295a 100644
--- a/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.h
+++ b/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.h
@@ -138,8 +138,7 @@
     class DeferredFence {
         public:
             DeferredFence()
-              : mMutex(),
-                mFences({Fence::NO_FENCE, Fence::NO_FENCE}) {}
+              : mFences({Fence::NO_FENCE, Fence::NO_FENCE}) {}
 
             void add(int32_t fenceFd) {
                 mFences.emplace(new Fence(fenceFd));
@@ -151,7 +150,6 @@
             }
 
         private:
-            mutable std::mutex mMutex;
             std::queue<sp<Fence>> mFences;
     };