reactor: cleanup write_some leftover
reactor: cleanup write_some leftover

As Avi noted, we don't need to call engine().writeable() in write_some,
as write_some is a method of the reactor.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>
Message-Id: <20200212214807.15685-1-7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

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