iostream: remove unused function
iostream: remove unused function

`output_stream::possibly_available()` is not used since long ago.
let's just ditch it.

Signed-off-by: Kefu Chai <cdc92d111bae0ed1ee3b3c8a1f849fcffc6b44f7@gmail.com>

scylladb/seastar,scylladb/seastar,scylladb/seastar