This is a regression after #15903. Before these changes
del_leaving_endpoint took IP as a parameter and did nothing
if it was called with a non-existent IP.
The problem was revealed by the dtest test_remove_garbage_members_from_group0_after_abort_decommission[Announcing_that_I_have_left_the_ring-]. The test was
flaky as in most cases the node died before the
gossiper notification reached all the other nodes. To make
it fail consistently and reproduce the problem one
can move the info log 'Announcing that I have' after
the sleep and add additional sleep after it in
storage_service::leave_ring function.
Fixes#16466Closesscylladb/scylladb#16508