gms: gossiper: don't use `seastar::async` in `mark_alive`
gms: gossiper: don't use `seastar::async` in `mark_alive`

Since `real_mark_alive` does not require `seastar::async`
now, we can eliminate the wrapping async call, as well.

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla