Remove apparently unused Computer/custom-jnlp.jelly

[jglick]

3eedda8 just said

TODO: work from home for the rest

Hmm. And then the link was deleted the next day in 10d9ca4, but not the file it linked to.

I can guess at what this was originally supposed to be—a place to edit JNLPLauncher.vmargs—but this was deleted in #6543 anyway.

Proposed changelog entries

• N/A

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least 2 approvals for the pull request and no outstanding requests for change
• Conversations in the pull request are over OR it is explicit that a reviewer does not block the change
• Changelog entries in the PR title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood
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• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[NotMyFault]

At least, no usage in jenkins plugins hosted in this org.

[NotMyFault]

This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!