Remove Mutex class from Doxygen docs. (#826)

Author: jonsimantov

app/src/include/firebase/internal/mutex.h

@@ -27,6 +27,8 @@
 
 namespace firebase {
 
+#if !defined(DOXYGEN)
+
 /// @brief A simple synchronization lock. Only one thread at a time can Acquire.
 class Mutex {
  public:
@@ -91,6 +93,8 @@ class MutexLock {
   Mutex* mutex_;
 };
 
+#endif  // !defined(DOXYGEN)
+
 }  // namespace firebase
 
 #endif  // FIREBASE_APP_SRC_INCLUDE_FIREBASE_INTERNAL_MUTEX_H_