SUNRPC: remove pointless test in unx_match()

As reported by Dan Carpenter, this test for acred->cred being set is
inconsistent with the dereference of the pointer a few lines earlier.

An 'auth_cred' *always* has ->cred set - every place that creates one
initializes this field, often as the first thing done.

So remove this test.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>