agentbase: remove flaky test
agentbase: remove flaky test

`test_start_with_pipeline_poller` do not test anything more than just
`test_start` and is actually flaky, as the pipeline can be refreshed in
between, making the number of calls uncertain.

Let's remove it.

Closes-Bug: #1621881
Change-Id: I2e43f1913c00f28e2a235936bf3d9f1a7cf6e5ad

openstack/ceilometer,openstack/ceilometer,ityaptin/ceilometer,ityaptin/ceilometer