Remove flaky assertion from FlowDurabilityTest.verifyExecutionRemoved

[jglick]

FlowDurabilityTest.testResumeBlocked and .testResumeBlockedAddedAfterRunStart frequently fail in PCT

java.lang.AssertionError
	at org.junit.Assert.fail(Assert.java:87)
	at org.junit.Assert.assertTrue(Assert.java:42)
	at org.junit.Assert.assertFalse(Assert.java:65)
	at org.junit.Assert.assertFalse(Assert.java:75)
	at org.jenkinsci.plugins.workflow.cps.FlowDurabilityTest.verifyExecutionRemoved(FlowDurabilityTest.java:219)
	at org.jenkinsci.plugins.workflow.cps.FlowDurabilityTest.verifyCompletedCleanly(FlowDurabilityTest.java:370)
	at org.jenkinsci.plugins.workflow.cps.FlowDurabilityTest.verifyFailedCleanly(FlowDurabilityTest.java:344)
	at org.jenkinsci.plugins.workflow.cps.FlowDurabilityTest$21.evaluate(FlowDurabilityTest.java:778)

java.lang.AssertionError
	at org.junit.Assert.fail(Assert.java:87)
	at org.junit.Assert.assertTrue(Assert.java:42)
	at org.junit.Assert.assertFalse(Assert.java:65)
	at org.junit.Assert.assertFalse(Assert.java:75)
	at org.jenkinsci.plugins.workflow.cps.FlowDurabilityTest.verifyExecutionRemoved(FlowDurabilityTest.java:219)
	at org.jenkinsci.plugins.workflow.cps.FlowDurabilityTest.verifyCompletedCleanly(FlowDurabilityTest.java:370)
	at org.jenkinsci.plugins.workflow.cps.FlowDurabilityTest.verifyFailedCleanly(FlowDurabilityTest.java:344)
	at org.jenkinsci.plugins.workflow.cps.FlowDurabilityTest$23.evaluate(FlowDurabilityTest.java:808)

I am not exactly sure why, but do we really need to be using reflection here? We do not really care about the internal field, so long as the visible effect is that the build is gone. ItemListenerImpl does cleanup (see jenkinsci/workflow-api-plugin#304).