Compaction group is responsible for deleting SSTables of "in-strategy"
compactions, i.e. regular, major, cleanup, etc.
Both in-strategy and off-strategy compaction have their completion
handled using the same compaction group interface, which is
compaction_group::table_state::on_compaction_completion(...,
sstables::offstrategy offstrategy)
So it's important to bring symmetry there, by moving the responsibility
of deleting off-strategy input, from manager to group.
Another important advantage is that off-strategy deletion is now throttled
and gated, allowing for better control, e.g. table waiting for deletion
on shutdown.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Closes#13432