[release-branch.go1.4] remove cmd/link

LGTM=dsymonds, minux
R=rsc, dsymonds, minux
CC=golang-codereviews
https://codereview.appspot.com/176910043
[release-branch.go1.4] remove cmd/link from nacl test zip

LGTM=dsymonds
R=rsc, dsymonds
CC=golang-codereviews
https://codereview.appspot.com/179830043