tests: lsa_async_eviction_test: Fix use after free due to at_exit() callback
tests: lsa_async_eviction_test: Fix use after free due to at_exit() callback

The callback will run after thread is destroyed. We don't really need
the stop feature, so for now just remove it.

scylladb/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,kjniemi/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,raphaelsc/scylla,kjniemi/scylla,raphaelsc/scylla,raphaelsc/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla