dockerfiles: remove npm npm-stretch and ci-jessie images
Those images were using firstname.lastname@example.org and used the npm Debian package from
Jessie to bootstrap it. With the upgrade to NodeJS 10, we went with a
npm snapshot in integration/npm.git.
The npm and npm-stretch are thus no more needed and are unused. They are
the last ones descending from ci-jessie so remove that image as well.