Don't return reference on copy assignment operators

We prefer to prevent using chained assignment (`T a = b = c = T();`) as this
can lead to confusing code and subtle bugs.

According to https://en.wikipedia.org/wiki/Assignment_operator_(C%2B%2B), C++
allows any arbitrary return type, so this is standard compliant.

This could be re-assessed if/when we have an actual need for a behavior more
akin to that of the C++ STL, for now this PR simply changes a handful of
cases which were inconsistent with the rest of the codebase (`void` return
type was already the most common case prior to this commit).

core/math/audio_frame.h

@@ -124,10 +124,9 @@ struct AudioFrame {
 		r = p_frame.r;
 	}
 
-	_ALWAYS_INLINE_ AudioFrame &operator=(const AudioFrame &p_frame) {
+	_ALWAYS_INLINE_ void operator=(const AudioFrame &p_frame) {
 		l = p_frame.l;
 		r = p_frame.r;
-		return *this;
 	}
 
 	_ALWAYS_INLINE_ operator Vector2() const {