RDMA/core: Remove FMR pool API

This ancient and unsafe method for memory registration is no longer used
by any RDMA based ULP. Remove the FMR pool API from the core driver.

Link: https://lore.kernel.org/r/4-v3-f58e6669d5d3+2cf-fmr_removal_jgg@mellanox.com


Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>