sstables/compaction: stop tracking exhausted input sstable in compaction_read_monitor
sstables/compaction: stop tracking exhausted input sstable in compaction_read_monitor

Motivation is that we want to release space for exhausted sstable and that
will only happen when all references to it are gone *and* that backlog
tracker takes the early replacement into account.

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

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