future: Delete the promise_base move assignment
future: Delete the promise_base move assignment

We never move assign just a promise_base, always a full promise.

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

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