[JENKINS-65308] Improve performance of Jenkins#trimLabels()

[res0nance]

See JENKINS-65308.

Instead of checking every node, when possible simply return fast instead of calculating the cached value.

This does alter its behaviour somewhat but not to the point its contracts are changed.

Tested with a simple test

@Before
    public void bef() throws Exception {
        for (int i = 0; i < 5; ++i) {
            j.createOnlineSlave(new LabelAtom("node" + i));
        }
    }

    @Test
    public void testTrim() {
        final long start = System.currentTimeMillis();
        for (int i = 0; i < 100000; ++i) {
            j.jenkins.trimLabels();
        }
        System.out.println("Test took: " + (System.currentTimeMillis()-start));
    }

New code Vs old time in ms:

Test Number	New	Old
1	1561	2253
2	1474	1998
3	1240	2185

Proposed changelog entries

• Entry 1: JENKINS-65308, Reduce queue-lock contention when creating or deleting nodes.

Proposed upgrade guidelines

N/A

Submitter checklist

• (If applicable) Jira issue is well described
• Changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developer, depending on the change). Examples
  • Fill-in the Proposed changelog entries section only if there are breaking changes or other changes which may require extra steps from users during the upgrade
• Appropriate autotests or explanation to why this change has no tests
• For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@mention

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 correct
• Proper changelog labels are set so that the changelog can be generated automatically
• If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (example)
• 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).

[MarkEWaite]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback.