(7.4.z) Remove unnecessary legacy security realm addition from RunAsWithElytronEJBContextPropagationTestCase

RunAsWithElytronEJBContextPropagationTestCase has a @BeforeClass method that adds a legacy security realm if one is not present and updates the existing one if it's there. The updated part is valid but the "add if not present part" is not needed and causes the test to fail on SE 17 when we no longer allow legacy security realm additions. So that bit of setup/teardown should be dropped.