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

@@ -30,19 +30,17 @@ func GoNop() {}
 func CgoCCodeSIGPROF() {
 	c := make(chan bool)
 	go func() {
-		for {
-			<-c
-			start := time.Now()
-			for i := 0; i < 1e7; i++ {
-				if i%1000 == 0 {
-					if time.Since(start) > time.Second {
-						break
-					}
+		<-c
+		start := time.Now()
+		for i := 0; i < 1e7; i++ {
+			if i%1000 == 0 {
+				if time.Since(start) > time.Second {
+					break
 				}
-				C.GoNop()
 			}
-			c <- true
+			C.GoNop()
 		}
+		c <- true
 	}()
 
 	var buf bytes.Buffer