FastThread: Remove unused NBLog variable

Test: Compile and verify NBLog
Change-Id: I145eb480415b076b7f75d266df35fc945563a364
diff --git a/services/audioflinger/FastThread.h b/services/audioflinger/FastThread.h
index bbb1d22..3f6b206 100644
--- a/services/audioflinger/FastThread.h
+++ b/services/audioflinger/FastThread.h
@@ -81,9 +81,7 @@
     struct timespec   mMeasuredWarmupTs;  // how long did it take for warmup to complete
     uint32_t          mWarmupCycles;  // counter of number of loop cycles during warmup phase
     uint32_t          mWarmupConsecutiveInRangeCycles;    // number of consecutive cycles in range
-    sp<NBLog::Writer> mDummyNBLogWriter;
-    NBLog::Writer*    mNBLogWriter;   // always non-nullptr: real NBLog::Writer* or
-                                      // mDummyNBLogWriter.get()
+    const sp<NBLog::Writer> mDummyNBLogWriter{new NBLog::Writer()};
     status_t          mTimestampStatus;
 
     FastThreadState::Command mCommand;