updateCollation.php: Switch back to using cl_from index for now

updateCollation.php: Switch back to using cl_from index for now

Using the cl_sortkey index instead (to reduce disruption to a live
site), as currently implemented, seems to have two serious problems:

* MySQL / MariaDB filesorts all rows that "sort above the given row
  [the last row of the previous batch]", not just a single category
  at a time until the row limit is reached.
* The current approach to pagination is broken in that it does not
  work with ENUM columns such as cl_type, causing 'file' rows to be
  skipped, or rows of any type to be repeated. See T119173.

This reverts part of commit a43f751cf6b6849e.

Bug: T58041
Change-Id: I619564e85b2122f249bdacc45d547b9ce1b3beb5