Changes:
* Remove obsolete targets which are unnecessary now that this repo no longer builds git submodules in a separate build process
* Remove the need for static analysis of 'ArtifactInfo' items
* Simplify how the code signing task is configured
* Remove unused repo tasks
* Remove duplicate lists of external dependencies and packages to be produced
* Remove obsolete build definition
* Remove obsolete build script parameters
* Add VisualStudioSetupOutputPath
* Update Extensions to 3.0.0-preview.18569.14
* Update EFCore to 3.0.0-preview.18569.2
* Update corefx to preview.18566.8
* Update core-setup to preview-27117-01