runtime: remove useless loop from CgoCCodeSIGPROF test program

I verified that the test fails if I undo the change that it tests for.

Updates #14732.

Change-Id: Ib30352580236adefae946450ddd6cd65a62b7cdf
Reviewed-on: https://go-review.googlesource.com/24151
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/runtime/testdata/testprogcgo/aprof.go

@@ -30,7 +30,6 @@ func GoNop() {}
 func CgoCCodeSIGPROF() {
 	c := make(chan bool)
 	go func() {
-		for {
 		<-c
 		start := time.Now()
 		for i := 0; i < 1e7; i++ {
@@ -42,7 +41,6 @@ func CgoCCodeSIGPROF() {
 			C.GoNop()
 		}
 		c <- true
-		}
 	}()
 
 	var buf bytes.Buffer