sstable: Unfriend sstable_directory class

It was only needed there for create_pending_deletion_log() method to get
private "_storage" from sstable. Now it's all gone and friendship can be
broken.

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

sstables/sstables.hh

@@ -526,7 +526,6 @@ private:
     }
 
     friend class sstable_stream_sink_impl;
-    friend class sstable_directory;
     friend class filesystem_storage;
     friend class s3_storage;
     friend class tiered_storage;