Fix expunging posts from S3 (fixup 4c4ef5c / #3180).

app/logical/backup_service.rb

@@ -2,4 +2,8 @@ class BackupService
   def backup(file_path, options = {})
     raise NotImplementedError.new("#{self.class}.backup not implemented")
   end
+
+  def delete(file_path, options = {})
+    raise NotImplementedError.new("#{self.class}.delete not implemented")
+  end
 end

app/logical/null_backup_service.rb

@@ -2,4 +2,8 @@ class NullBackupService
   def backup(file_path, options = {})
     # do nothing
   end
+
+  def delete(file_path, options = {})
+    # do nothing
+  end
 end