tests/avocado/migration: Remove the malfunctioning s390x tests

tests/avocado/migration: Remove the malfunctioning s390x tests

The tests from tests/avocado/migration.py do not work at all
on s390x - the bios shuts down immediately when it cannot find
a boot disk, so there is nothing left to migrate here. For doing
a proper migration test, we would need a proper payload, but we
already do such tests in the migration *qtest*, so it is unnecessary
to redo such a test here, thus let's simply remove this test.

Message-Id: <20230721164346.10112-1-thuth@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>