pending_replacement list is used by incremental compaction to
communicate to other ongoing compactions about exhausted sstables
that must be replaced in the sstable set they keep for tombstone
GC purposes.
Reshape doesn't enable tombstone GC, so that list will not
be cleared, which prevents incremental compaction from releasing
sstables referenced by that list. It's not a problem until now
where we want reshape to do incremental compaction.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>