build: remove duplicate dependency in Make.cmd

R=golang-dev, dfc, rsc1, rsc
CC=golang-dev
https://golang.org/cl/4274048

src/Make.cmd

@@ -34,8 +34,6 @@ testpackage: _test/main.a
 testpackage-clean:
 	rm -f _test/main.a _gotest_.$O
 
-testpackage: _test/main.a
-
 _test/main.a: _gotest_.$O
 	@mkdir -p _test
 	rm -f $@