Delete all fts3 index data the table becomes empty. Previously,

Delete all fts3 index data the table becomes empty.  Previously,
deleting all rows from an fts3 table would leave a bunch of index data
describing the terms of the original data, plus deletions of those
terms, perhaps with some amount of it merged together so the deletions
knocked out the originals.  Even when all rows were deleted that
original data would hang out, though eventually it would mostly be
overwritten if new data contained the same set of terms. (CVS 5413)

FossilOrigin-Name: 8b872e426091d9ef108e52dbec0d968ed7452907