build: remove support for VS2015

PR-URL: https://github.com/nodejs/node/pull/16969
Refs: https://github.com/nodejs/node/pull/16868


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>