[infra] Remove .packages and pubspec from js_runtime

Change-Id: I8382da009d20200a26f4499c00b736a7240109ec
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/411303
Reviewed-by: Nate Biggs <[email protected]>
Commit-Queue: Slava Egorov <[email protected]>

Author: mraleph