Fix Coverity reports of uninitialized scalar variable

Fixes most current reports on Coverity Scan of uninitialized scalar
variable (CWE-457): https://cwe.mitre.org/data/definitions/457.html

These happen most of the time (in our code) when instanciating structs
without a constructor (or with an incomplete one), and later returning
the instance. This is sometimes intended though, as some parameters are
only used in some situations and should not be double-initialized for
performance reasons (e.g. `constant` in ShaderLanguage::Token).

modules/visual_script/visual_script.h

@@ -374,12 +374,10 @@ class VisualScriptInstance : public ScriptInstance {
 		int node;
 		int max_stack;
 		int trash_pos;
-		int return_pos;
 		int flow_stack_size;
 		int pass_stack_size;
 		int node_count;
 		int argument_count;
-		bool valid;
 	};
 
 	Map<StringName, Function> functions;