Remove v4.x and v9.x labels from the list

Node.js 4 and 9 are both EOL. I'd like to clean up the label list a bit and remove labels related to those release lines.

v4.x labels: `v4.x`, `land-on-v4.x`, `dont-land-on-v4.x`, `lts-watch-v4.x`
v9.x labels: `v9.x`, `land-on-v9.x`, `dont-land-on-v9.x`, `backported-to-v9.x`, `backport-requested-v9.x`

Refs: https://github.com/nodejs/node/issues/16407