[libc] Remove ballot on slab find (#176606)

Summary:
This negatively impacts performance, while the other changes in the
initial PR slightly improved it. This was originally done to make Volta
independent thread scheduling work, but that doesn't seem to work
correctly all the time either so we should make this faster.
GitOrigin-RevId: 739e997c3e7ba4cc8f467d61d26ae64e60e81d70