don't flush the tree when evaluating the *-argument conditionals

This is a follow-up to PR scp-fs2open#2942.  Given that the short-circuiting is now explicitly skipped for node trees containing the special argument, it should no longer be necessary to flush the conditional branch of the sexp tree.  (The .cache field is already skipped for special-arg nodes.)

The sexp tree is still explicitly flushed for `every-time` and `every-time-argument`.

code/parse/sexp.cpp

@@ -9542,9 +9542,6 @@ int test_argument_nodes_for_condition(int n, int condition_node, int *num_true,
 		// only eval this argument if it's valid
 		if (Sexp_nodes[n].flags & SNF_ARGUMENT_VALID)
 		{
-			// flush conditional to avoid short-circuiting
-			flush_sexp_tree(condition_node);
-
 			// evaluate conditional for current argument
 			Sexp_replacement_arguments.emplace_back(Sexp_nodes[n].text, n);
 			val = eval_sexp(condition_node);
@@ -9626,9 +9623,6 @@ int test_argument_vector_for_condition(const SCP_vector<std::pair<char*, int>> &
 	{
 		// since we can't see or modify the validity, assume all are valid
 		{
-			// flush conditional to avoid short-circuiting
-			flush_sexp_tree(condition_node);
-
 			// evaluate conditional for current argument
 			Sexp_replacement_arguments.push_back(argument);
 			val = eval_sexp(condition_node);
@@ -9838,9 +9832,8 @@ int eval_random_of(int arg_handler_node, int condition_node, bool multiple)
 	val = SEXP_FALSE;
 	if (Sexp_nodes[n].flags & SNF_ARGUMENT_VALID)
 	{
-		// flush stuff
+		// ensure special argument list is empty
 		Sexp_applicable_argument_list.clear_nesting_level();
-		flush_sexp_tree(condition_node);
 
 		// evaluate conditional for current argument
 		Sexp_replacement_arguments.emplace_back(Sexp_nodes[n].text, n);
@@ -9891,9 +9884,8 @@ int eval_in_sequence(int arg_handler_node, int condition_node)
 	// Only execute if the argument is valid (if all nodes were invalid we would still reach this point)
 	if (Sexp_nodes[n].flags & SNF_ARGUMENT_VALID)
 	{
-		// flush stuff
+		// ensure special argument list is empty
 		Sexp_applicable_argument_list.clear_nesting_level();
-		flush_sexp_tree(condition_node);
 
 		// evaluate conditional for current argument
 		Sexp_replacement_arguments.emplace_back(Sexp_nodes[n].text, n);