src: unexport node_isolate

Commit 0bba5902 accidentally (or maybe erroneously) added node_isolate
to src/node.h and src/node_object_wrap.h.

Undo that, said variable is not for public consumption. Add-on authors
should use v8::Isolate::GetCurrent() instead.

I missed that while reviewing. Mea culpa.

Fixes #5639.