vinyl: drop useless vy_read_view->is_aborted flag

It is supposed to abort an iterator that read a statement reverted after
a failed WAL write. However, we will abort such an iterator anyway - see
vy_tx_abort_readers(). So let's drop the useless flag.

It turned out that we never tested such a case (try grepping "read view
is aborted" error). So let's also add a functional test for it.