remove unused future import in file script
remove unused future import in file script

python-future lib is not part of the vpython spec, so this import is
causing build failure like:
https://logs.chromium.org/logs/chromeos/led/chromeos-staging_chromeos-bot.iam.gserviceaccount.com/86d6ff4b3a94f1f6d9aa642f1105cd80e918bb20832a96cfe576a0e9d44ce170/+/steps/upload_prebuilts/0/steps/upload_prebuilt_blobs_to_GS/0/steps/link_files_to_upload/0/stdout

R=yuanjunh

Change-Id: Ic1aa0444a386348979c81478bf12c54a48a5d8b7
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/3152159
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Commit-Queue: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Auto-Submit: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Reviewed-by: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>

luci/recipes-py,luci/recipes-py