future: Delete unused do_void_futurize_helper instantiation
future: Delete unused do_void_futurize_helper instantiation

With the recent patches already on master do_void_futurize_helper is
only instantiated with void. This deletes the future<> instantiation
and turns the class template into a plain class.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200324220326.235414-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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