vinyl: do not call vy_scheduler_complete_dump on index deletion

Currently, vy_scheduler_remove_mem() calls vy_scheduler_complete_dump()
if vy_scheduler_dump_in_progress() returns false, but the latter doesn't
necessarily mean that the dump has just been completed. The point is
that vy_scheduler_remove_mem() is called not only for a memory tree that
has just been dumped to disk, but also for all memory trees of a dropped
index, i.e. dropping an index when there's no dump in progress results
in vy_scheduler_complete_dump() invocation. This doesn't do any harm
now, but looks ugly. Besides, I'm planning to account dump bandwidth in
vy_scheduler_complete_dump(), which must only be done on actual dump
completion.