[FIX] Remove duplicate "To Purchase" filter.

Duplicate product filters "To Purchase" and "Can be Purchased". Same
search criteria and same name, so removing one doesn't affect
people who were referencing it (e.g. with `search_default_*` key in context).

Left the one named "Can be Purchased" because consistent with the
equivalent sale filter "Can be Sold".

Fixes #2718.