C#+iOS: Fix crash at exit for passing NULL domain to mono_jit_cleanup

modules/mono/mono_gd/gd_mono.cpp

@@ -1263,7 +1263,6 @@ GDMono::~GDMono() {
 
 		project_assembly = NULL;
 
-		root_domain = NULL;
 		scripts_domain = NULL;
 
 		// Leave the rest to 'mono_jit_cleanup'