Executor: Remove LegacyInstanceReuse strategy (#1486)

It seems the old strategy have been depracted more than one year. 
So maybe it's time to clean up old strategy for wasm executor.


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polkadot address: 15ouFh2SHpGbHtDPsJ6cXQfes9Cx1gEFnJJsJVqPGzBSTudr

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Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Koute <koute@users.noreply.github.com>

substrate/client/executor/benches/bench.rs

@@ -150,13 +150,6 @@ fn bench_call_instance(c: &mut Criterion) {
 	let _ = env_logger::try_init();
 
 	let strategies = [
-		(
-			"legacy_instance_reuse",
-			Method::Compiled {
-				instantiation_strategy: InstantiationStrategy::LegacyInstanceReuse,
-				precompile: false,
-			},
-		),
 		(
 			"recreate_instance_vanilla",
 			Method::Compiled {