IB/pvrdma: Remove unused function

The function pvrdma_idx_ring_is_valid_idx is not in used so let's remove
it.

Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Acked-by: Adit Ranadive <aditr@vmware.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>