compaction: abort if create_new_sstable is called from resharding
compaction: abort if create_new_sstable is called from resharding

I am about to get rid of the _shard attribute in the compaction object,
as I will create different streams of writers for different shards.

In preparation for that, remove the arbitrary _shard reference. Raphael
confirms that resharding should never be calling this, as this method is
used exclusively for garbage collection component of run-based
compaction. Therefore we'll just throw in this case and remove the shard
reference.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

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