Delete the data file if it has no valid data

Currently, when we execute "delete from root.sg.* where time < T", we write modifications besides the deleted TsFile.

It's better to delete the file that does not have valid data.

Start by tracing how `delete from root.sg.* where time < T` handles the affected TsFile and writes modifications. Determine how the system identifies that a file has no valid data, then verify that such a file is deleted rather than retained with modification records.

Written by the indexing model from the issue text.