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).

modules/mono/godotsharp_dirs.cpp

@@ -229,9 +229,6 @@ private:
 #endif
 	}
 
-	_GodotSharpDirs(const _GodotSharpDirs &);
-	_GodotSharpDirs &operator=(const _GodotSharpDirs &);
-
 public:
 	static _GodotSharpDirs &get_singleton() {
 		static _GodotSharpDirs singleton;