vinyl: zap sdc struct

sdc is a buffer allocated in each worker thread and passed to worker
functions where it can be used to avoid malloc. Turns out it is only
used in vy_range_redistribute_index(), where only sdc->b (vy_buf) is
needed. And even that is dubious, because it isn't a hot path, so it'd
be OK to call malloc there directly. So let's kill this sdc struct.