Do not pack ts dimension in data importer

`OpenMLDB` will extract the value of ts col from encoded value https://github.com/4paradigm/OpenMLDB/issues/529, so data importer need not pack ts dimension

The issue references a change in the data importer to stop packing the ts dimension, as ts column values will be extracted from encoded values per issue #529. Look for the data importer code, likely in src/importer or similar directories, to find where ts dimension packing occurs. Understand the encoding extraction logic from the linked issue to ensure the importer no longer duplicates this packing. Verify changes by running any existing tests related to data import and ts column handling.

索引モデルが issue の本文から書いたものです。