HTCONDOR-2988 mark Classad::Remove as [[nodiscard]]

[GregThain]

And remove a memory leak that it found.

<Insert PR description here, and leave checklist below for code review.>

HTCondor Pull Request Checklist for internal reviewers

• Verify that (GitHub thinks) the merge is clean. If it isn't, and you're confident you can resolve the conflicts, do so. Otherwise, send it back to the original developer.
• Verify that the related Jira ticket exists and has a target version number and that it is correct.
• Verify that the Jira ticket is in review status and is assigned to the reviewer.
• Verify that the Jira ticket (HTCONDOR-xxx) is mentioned at the beginning of the title. Edit it, if not
• Verify that the branch destination of the PR matches the target version of the ticket
• Check for correctness of change
• Check for regression test(s) of new features and bugfixes (if the feature doesn't require root)
• Check for documentation, if needed (documentation build logs)
• Check for version history, if needed
• Check BaTLab dashboard for successful build (https://batlab.chtc.wisc.edu/results/workspace.php) and test for either the PR or a workspace build by the developer that has the Jira ticket as a comment.
• Check that each commit message references the Jira ticket (HTCONDOR-xxx)

After the above

• Hit the merge button if the pull request is approved and it is not a security patch (security changes require 2 additional reviews)
• If the pull request is approved, take the ticket out of review state
• Assign JIRA Ticket back to the developer

[JaimeFrey]

LGTM