srml-contracts: Apply contract removals immediately (#3417)

* Add ability to destroy a contract in the overlay.

* Don't allow contracts to be destroyed in recursive execution.

* Tests for contract self-destruction.

* Don't allow constructor to exit with insufficient balance.

* Remove dead code.

* Bump node runtime spec version.

substrate/srml/contracts/src/wasm/runtime.rs

@@ -70,10 +70,6 @@ impl<'a, E: Ext + 'a> Runtime<'a, E> {
 			special_trap: None,
 		}
 	}
-
-	fn memory(&self) -> &sandbox::Memory {
-		&self.memory
-	}
 }
 
 pub(crate) fn to_execution_result<E: Ext>(