Remove valueless recurring timer last-trigger test

[ehelms]

Why are you introducing these changes? (Problem description, related links)

The test provids no unique value: test_foreman_recurring_timer_next_trigger
already proves each timer is scheduled to fire, and
test_foreman_recurring_timer_execution already proves the service and its
container plumbing actually run. Asserting that systemd recorded a past
trigger timestamp verifies nothing about the deployment's correctness.

See failures: https://github.com/theforeman/foreman-oci-images/actions/runs/33086235201/job/98566514040?pr=91

What are the changes introduced in this pull request?

• Remove the test_foreman_recurring_timer_last_trigger test, which was
  deploy-timing-dependent and duplicated coverage already provided by the
  next-trigger and execution tests.

How to test this pull request

Steps to reproduce:

• Deploy a VM with recurring timers enabled and run the recurring timer
  tests:
  python -m pytest tests/feature/foreman/base_test.py -vv -k recurring
• Confirm the suite passes on a fast fresh deploy (no pre-pulled images, no
  re-deploy) — previously test_foreman_recurring_timer_last_trigger[monthly]
  would fail in that scenario.

Checklist

• Tests added/updated (if applicable)
• Documentation updated (if applicable)

[ehelms]

@archanaserver one more for you to consider