[FIX] Remove duplicate "Can be purchased" filter.

Duplicate product filter "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, closes #2864