vinyl: remove pointless assertion from vy_stmt_new_surrogate_delete

For some reason, vy_stmt_new_surrogate_delete() checks that the source
tuple has all fields mandated by the space format (min_field_count).
This is pointless, because to generate a surrogate DELETE statement, we
don't need all tuple fields - if a field is absent it will be replaced
with NULL. We haven't stepped on this assertion, because we always
create surrogate DELETEs from full tuples. However, to implement #2129
we need to be able to create surrogate DELETEs from tuples that only
have indexed fields. So let's remove this assertion.

Needed for #2129