refactor: remove postinstall patch

It appears they renamed postinstall.js to postinstall.mjs and removed
the use of `rimraf` which means our patch is no longer needed! 🎉

microsoft/vscode@b0e8554

Author: Joe Previte

patches/series

@@ -12,7 +12,6 @@ proxy-uri.diff
 display-language.diff
 github-auth.diff
 unique-db.diff
-post-install.diff
 log-level.diff
 local-storage.diff
 service-worker.diff