v8::debug::GetDebugContext is going away.

[hashseed]

If I remember correctly we agreed on delaying the removal of GetDebugContext until Node 9. I would like to remove the API and more importantly the legacy code backing it. That means that Node cannot use it anymore though. It currently still does.

Given the success of the DevTools protocol I think we can remove it. Can we advance this somehow so that by Node 10 it's not exposed to add-ons anymore?

[hashseed]

/cc @ofrobots

[bnoordhuis]

I removed the last vestiges in core in November, see #13295. Node 10 won't be using it.

I'm okay with making it inaccessible to add-ons (in node 10); otherwise it might hold us back trying to upgrade V8 mid-cycle.