xprtrdma: Remove stale comment

The comment hasn't been accurate for several years.

Signed-off-by: Chuck Lever <[e-mail]>
Signed-off-by: Anna Schumaker <[e-mail]>

net/sunrpc/xprtrdma/xprt_rdma.h

···
  *
  * An external memory region is any buffer or page that is registered
  * on the fly (ie, not pre-registered).
- *
- * Each rpcrdma_buffer has a list of free MWs anchored in rb_mrs. During
- * call_allocate, rpcrdma_buffer_get() assigns one to each segment in
- * an rpcrdma_req. Then rpcrdma_register_external() grabs these to keep
- * track of registration metadata while each RPC is pending.
- * rpcrdma_deregister_external() uses this metadata to unmap and
- * release these resources when an RPC is complete.
  */
 enum rpcrdma_frwr_state {
 	FRWR_IS_INVALID,	/* ready to be used */