The struct mmu_rb_ops function pointers .insert, .invalidate were only
used to increment and decrement struct sdma_mmu_node.refcount.

With the deletion of struct sdma_mmu_node.refcount and the addition of
struct mmu_rb_node.refcount  these function pointers are not called and
there are no implementations of them. So it is safe to delete these from
struct mmu_rb_ops.

Link: https://lore.kernel.org/r/168451526508.3702129.8677714753157495310.stgit@awfm-02.cornelisnetworks.comSigned-off-by: Brendan Cunningham <bcunningham@cornelisnetworks.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>