commitlog: remove _needed_size
commitlog: remove _needed_size

It is mostly an optimization, and while it makes sense in this context,
it won't soon as we'll stop waiting for the current cycle specifically
to finish.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

avikivity/scylla,raphaelsc/scylla,raphaelsc/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla