vinyl: zap vy_run_iterator->curr_stmt_pos

vy_run_iterator_get() remembers the position of the last statement
returned by the iterator in curr_stmt_pos. It then uses it to skip
a disk read in case it is called again for the same iterator position.
The code is left from the time when iterators had public virtual
method 'get', which could be called several times without advancing
the iterator. Nowadays, vy_run_iterator_get() is never called twice
for the same iterator position (check coverity scan) so we can zap
this logic.