xprtrdma: Remove REGISTER memory registration mode

All kernel RDMA providers except amso1100 support either MTHCAFMR
or FRMR, both of which are faster than REGISTER.  amso1100 can
continue to use ALLPHYSICAL.

The only other ULP consumer in the kernel that uses the reg_phys_mr
verb is Lustre.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>