decoder/Control: remove unused methods Lock(), Unlock()

src/decoder/Control.hxx

@@ -193,20 +193,6 @@ public:
 		thread.Start();
 	}
 
-	/**
-	 * Locks the object.
-	 */
-	void Lock() const noexcept {
-		mutex.lock();
-	}
-
-	/**
-	 * Unlocks the object.
-	 */
-	void Unlock() const noexcept {
-		mutex.unlock();
-	}
-
 	/**
 	 * Signals the object.  This function is only valid in the
 	 * player thread.  The object should be locked prior to