compaction: introduce compaction_read_monitor_generator::remove_exhausted_sstables()
compaction: introduce compaction_read_monitor_generator::remove_exhausted_sstables()

This new function makes it easier to remove monitor of exhausted
sstables.

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

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