pcm/Volume: remove assert() from destructor

While this assert() was useful when we had to track the object's state
manually, there was no practical purpose other than verifying old
code, and it complicates our new C++ code.

src/pcm/Volume.hxx

@@ -77,12 +77,6 @@ public:
 #endif
 	}
 
-#ifndef NDEBUG
-	~PcmVolume() {
-		assert(format == SampleFormat::UNDEFINED);
-	}
-#endif
-
 	unsigned GetVolume() const {
 		return volume;
 	}