net/tls: explicitly ignore ready future during shutdown
net/tls: explicitly ignore ready future during shutdown

The ready future it not handled in any way, but in case it is
exceptional a warning is printed in the log. Explicitly ignoring the
future to silence the warning.

Message-Id: <cc15f9a1dc9528c23c4b0f16375fe7f93e57a035@scylladb.com>

avikivity/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,dreamsxin/seastar,cloudius-systems/seastar,cloudius-systems/seastar,avikivity/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar