rpc: remove unnecessary assignment

We're just about to kfree() gss_auth, so there's no point to setting any
of its fields.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/auth_gss/auth_gss.c

@@ -650,7 +650,6 @@ static void
 gss_free(struct gss_auth *gss_auth)
 {
 	rpc_unlink(gss_auth->dentry);
-	gss_auth->dentry = NULL;
 	gss_mech_put(gss_auth->mech);
 
 	kfree(gss_auth);