compaction: kill useless on_skipped_expired_sstable()
compaction: kill useless on_skipped_expired_sstable()

It was introduced by commit 5206a97915 because fully expired sstable
wouldn't be registed and therefore could be never removed from backlog
tracker. This is no longer possible as table is now responsible for
removing all input sstables. So let's kill on_skipped_expired_sstable()
as it's now only boilerplate we don't need.

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

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