Remove workaround in CreateItemTest#duplicate_item_name_displays_error

[basil]

Starting with jenkinsci/jenkins#9111 by @janfaracik in 2.453, CreateItemTest#duplicate_item_name_displays_error fails with

java.lang.AssertionError
	at org.junit.Assert.fail(Assert.java:87)
	at org.junit.Assert.assertTrue(Assert.java:42)
	at org.junit.Assert.assertTrue(Assert.java:53)
	at core.CreateItemTest.duplicate_item_name_displays_error(CreateItemTest.java:47)

This regression is tracked in core as JENKINS-73034.

This regression destabilized this test suite, as described in #1545. A workaround was added in this repository in #1547. As part of fixing JENKINS-73034, this workaround should be removed.