Remove unused JavaScript file (lib/bounce.js)

Why is this change being made?

This unused JavaScript file is orphan so ready to remove.

I am not sure if this file is a third-party and where it came from.

Author Checklist

• Check list for being orphan
  • https://sourcegraph.com/search?q=repo:gitlab.com/gitlab-com/www-gitlab-com+bounce%5C.js&patternType=literal&case=yes
• History: originally introduced in d69bc4c1 as the first commit of the repo on 2013-07-05 by @sytses and renamed in 4f909c60, and then renamed in 87300a7b and removed once, but reverted in 0d9d20aa

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• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per [say-why-not-just-what][transparency]
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in [#mr-buddies][mr-buddies-slack].
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.