vinyl: fix index use-after-free in scheduler

We don't remove dead index's ranges from the scheduler heap after we
scheduled a drop task for it - we only do it from the drop task itself.
As a result, we can happily go and schedule another task for the index
being already scheduled for destruction. This can result in the
index->ref == 1 assertion being violated in vy_task_drop_execute() or
memory corruption induced by use-after-free of a vy_index struct.