monitoring: mention PyYAML only once in requirements

monitoring: mention PyYAML only once in requirements

Following error occurs while running "sudo install-deps.sh" -
ERROR: Double requirement given: PyYAML==6.0 (from -r requirements-lint.txt (line 5)) (already in pyyaml (from -r requirements-alerts.txt (line 1)), name='PyYAML')

PyYAML is mentioned twice as a requirement. It is mentioned once in both
the following files -
monitoring/ceph-mixin/requirements-lint.txt
monitoring/ceph-mixin/requirements-alerts.txt

These requirements were added in commits
44d3e4c264506154373ffaeb13d6c924c580e6b5 and
4750ac0d7766a8a089adf073415af0ac0d3f81d9.

Fixes: https://tracker.ceph.com/issues/54185
Signed-off-by: Rishabh Dave <ridave@redhat.com>