remove INSTANCE_CACHE from dynamic_link branch

[loloicci]

INSTANCE_CACHE is a thread-local variable used in some tests for old dynamic link way.

cosmwasm/packages/vm/src/testing/mock.rs

Lines 43 to 48 in 9161e25

	thread_local! {
	// INSTANCE_CACHE and MODULE_CACHE are intended to replace wasmvm's cache layer in the mock.
	// Unlike wasmvm, you have to initialize them yourself in the place where you test the dynamic call.
	pub static INSTANCE_CACHE: RwLock<HashMap<String, RefCell<MockInstance>>> = RwLock::new(HashMap::new());
	pub static MODULE_CACHE: RwLock<HashMap<String, RefCell<Module>>> = RwLock::new(HashMap::new());
	}

But, it is not used after #273. So This should be removed.

[loloicci]

closed via #296