the corresponding implementation of operator<< was dropped in
a40d3fc25b, so there is no needs to
keep this friend declaration anymore.
also, drop `include <ostream>`, as this header does not reference
any of the ostream types with the change above.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes scylladb/scylladb#17743