R1:fb3f423279b2

Remove broken and unfixable "prefix" ngram behavior

Summary:
Ref T13501. The older ngram code has some "prefix" behavior that tries to handle cases where a user issues a very short (one or two character) query.

This code doesn't work, presumably never worked, and can not be made to work (or, at least, I don't see a way, and am fairly sure one does not exist).

If the user searches for "xy", we can find trigrams in the form "xy*" using the index, but not in the form "*xy". The code makes a misguided…