sstables: Remove duplicated friend declaration
sstables: Remove duplicated friend declaration

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla