Remove duplicate (broken) definition of `assert_ArrayBuffer`

[backes]

This was removed from the upstream test in https://github.com/WebAssembly/spec/commit/cc2d59bd56e5342e3c1834a7699915f8b67fc29c.

The assert_ArrayBuffer helper is now defined in the assertions.js file. The version there does not have the problem with self.

This problem was uncovered when importing the threads JS tests into V8, see https://crbug.com/449581914.