build: don't build goplay in run.rc.

Fix plan 9 build.

TBR=rsc
R=golang-codereviews
CC=golang-codereviews
https://golang.org/cl/100880047

src/run.rc

@@ -48,10 +48,6 @@ fn xcd {
 	cd $"GOROOT/src/$1
 }
 
-echo
-echo '#' ../misc/goplay
-go build ../misc/goplay
-
 echo
 echo '#' ../test/bench/go1
 go test ../test/bench/go1