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).

servers/physics_server_3d.h

@@ -965,10 +965,9 @@ class PhysicsServer3DManager {
 				name(p_ci.name),
 				create_callback(p_ci.create_callback) {}
 
-		ClassInfo &operator=(const ClassInfo &p_ci) {
+		void operator=(const ClassInfo &p_ci) {
 			name = p_ci.name;
 			create_callback = p_ci.create_callback;
-			return *this;
 		}
 	};