misc/nacl: exclude cmd/link from the test zip.

It does not appear to be necessary, and cmd/link does not appear in release branches.

LGTM=rsc
R=adg, rsc
CC=golang-codereviews
https://golang.org/cl/176900044

misc/nacl/testzip.proto

@@ -18,9 +18,6 @@ go	src=..
 				gofmt_test.go
 				testdata
 					+
-			link
-				testdata
-					+
 		archive
 			tar
 				testdata