compaction_manager: simplify rewrite_sstables()
compaction_manager: simplify rewrite_sstables()

as rewrite_sstables() switched to coroutine, it can be simplified
by not using smart pointers to handle lifetime issues.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

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