test: remove test-diagnostics-channel-memory-leak.js

There is no reliable way to detect this leak because:

1. We cannot reliably get a reference to the channel from the
  API to detect finalization without creating another strong reference.
2. This test does gc() and then checks memory usage - however the
  use of gc() disables code aging which can actually lead to increased
  memory usage overall, as it is not intended to be used to lower
  memory usage in the first place.
3. The implementation of diagnostics channels relies on ephemeron gc
  which is inefficient, it's not reliable to use the typical "create
  a lot of objects and see if it crashes" trick to check leaks.

To avoid flakiness in the CI, it's better to remove an unreliable
test altogether.

Author: joyeecheung