xprtrdma: Remove last ib_reg_phys_mr() call site

All HCA providers have an ib_get_dma_mr() verb. Thus
rpcrdma_ia_open() will either grab the device's local_dma_key if one
is available, or it will call ib_get_dma_mr(). If ib_get_dma_mr()
fails, rpcrdma_ia_open() fails and no transport is created.

Therefore execution never reaches the ib_reg_phys_mr() call site in
rpcrdma_register_internal(), so it can be removed.

The remaining logic in rpcrdma_{de}register_internal() is folded
into rpcrdma_{alloc,free}_regbuf().

This is clean up only. No behavior change is expected.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Devesh Sharma <devesh.sharma@avagotech.com>
Reviewed-By: Sagi Grimberg <sagig@mellanox.com>
Tested-by: Devesh Sharma <devesh.sharma@avagotech.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>