tests: test-galera-upgrades.sh: Remove the exit trap
tests: test-galera-upgrades.sh: Remove the exit trap

We are already collecting logs as part of the post-run playbook so we
don't need to do that as part of the exit trap. This avoid collecting
and compressing the logs twice.

Change-Id: If6f0adb93bcc6e2cae3885017c58159f3dca1772

openstack/openstack-ansible-galera_server,os-cloud/openstack-ansible-galera_server,openstack/openstack-ansible-galera_server,os-cloud/openstack-ansible-galera_server