release: remove pmc deployment workflow

The Debian package for each microsoft/git release is currently deployed to
packages.microsoft.com. The team that manages this feed is currently
migrating to a new "vNext Publishing API" [1]. Because the new API has
several administrative pre-requisites and the maintainers believe
consumption of microsoft/git from this feed to be negligible, the decision
was made to cease deploying to packages.microsoft.com rather than migrate
to the new API. This change removes the workflow that handled this
deployment.

1: https://microsoft.sharepoint.com/teams/PMC/SitePages/API-An.aspx