Use natvis files from vs-debugtools

As recently discussed on the qt-development mailing list, we should move the natvis files to a central repo used by the VS and the VS Code extensions. There was one created for that purpose a while ago: qt-labs/vs-debugtools.

Since this repo is in the same namespace as qt-labs/vstools, adding a git submodule is as simple as

git submodule add ../vs-debugtools

The task is to remove our (two!) copies of natvis files from vstools and make qt-labs/vs-debugtools the single source of truth.