[vector] Change default branch and remove stale branches

[Paul Sandoz]

> On Sep 16, 2020, at 7:15 AM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:
> 
> 
>> (The following stale branches will remain: codeSnippet, vectorSnippets)
> 
> I think they can be removed as well. There hasn't been any activity there for a long time and they'll stay in mercurial repo (panama/dev) anyway if there's a need.

Ok, I shall remove ‘em too.

Paul.