Remove SquidUpdate::newFromLinksTo()

This has been broken since r9199 / e0ce5a322e41; a DB query error will occur
because the links table no longer exists, and even if the table were to
exist, the pl_* columns would not exist in that table.

There is no usage in core or Gerrit-hosted extensions.

Change-Id: I4922f85417bfb4d40f08662ba100ab5b78edd5fb