xprtrdma: Remove include for linux/prefetch.h

Clean up. This include should have been removed by
commit 23826c7aea ("xprtrdma: Serialize credit accounting again").

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Devesh Sharma <devesh.sharma@broadcom.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/verbs.c

@@ -49,7 +49,6 @@
 
 #include <linux/interrupt.h>
 #include <linux/slab.h>
-#include <linux/prefetch.h>
 #include <linux/sunrpc/addr.h>
 #include <linux/sunrpc/svc_rdma.h>