xprtrdma: Remove rl_mr field, and the mr_chunk union

Clean up: Since commit 0ac531c1 ("xprtrdma: Remove REGISTER
memory registration mode"), the rl_mr pointer is no longer used
anywhere.

After removal, there's only a single member of the mr_chunk union,
so mr_chunk can be removed as well, in favor of a single pointer
field.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>