Add method to easily remove Vertex

[brean]

Hi,

I think it should be possible to easily delete a Vertex when I have its instance. The API might look like this:

class Foo(Vertex):
   ...

async def remove_all_foo():
    session = await app.session()
    foos = session.traversal(Foo)
    async for foo in foos:
        await foo.remove()

[brean]

looks at least more intuitive then await session.remove_vertex(foo)