decoder/Control: caller must lock mutex for Seek()

src/decoder/DecoderControl.cxx

@@ -128,8 +128,6 @@ DecoderControl::Stop() noexcept
 void
 DecoderControl::Seek(SongTime t)
 {
-	const std::lock_guard<Mutex> protect(mutex);
-
 	assert(state != DecoderState::START);
 	assert(state != DecoderState::ERROR);
 

src/decoder/DecoderControl.hxx

@@ -387,6 +387,8 @@ public:
 
 	/**
 	 * Throws #std::runtime_error on error.
+	 *
+	 * Caller must lock the object.
 	 */
 	void Seek(SongTime t);