refactor: remove unused INSTNACE_CACHE

[loloicci]

Description

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.

Closes #295

Types of changes

• Bug fix (changes which fixes an issue)
• New feature (changes which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• ETC (build, ci, docs, perf, refactor, style, test)

Checklist

• I followed the contributing guidelines.
• I have updated the documentation accordingly. (not needed)
• I have added tests to cover my changes. (not needed)
• The PR title and commits are followed conventional commit form.

[dudong2]

INSTANCE_CACHE is being used in packages/vm/benches/main.rs. However, it hasn't been removed in this PR. I think it didn't throw an error because the benchmark doesn't work in CI.

[loloicci]

@dudong2 Thank you for finding it. Fixing for the benchmark is done in #294 and it contains other fixes. So if we remove INSTANCE_CACHE from vm::benches::main, the benchmark will fail. So, I will leave the fix of the benchmark including removing INSTANCE_CACHE at #294.