junit: Simplify the formatOutput function

This function was used in the past to perform some operations on the
individual output lines before joining them together. However, now it's
only used to combine the collection of lines into a single output
string.

junit/junit.go

@@ -264,14 +264,9 @@ func formatBenchmarkTime(d time.Duration) string {
 	return fmt.Sprintf("%.9f", d.Seconds())
 }
 
-// formatOutput trims the test whitespace prefix from each line and joins all
-// the lines.
+// formatOutput combines the lines from the given output into a single string.
 func formatOutput(output []string, indent int) string {
-	var lines []string
-	for _, line := range output {
-		lines = append(lines, line)
-	}
-	return strings.Join(lines, "\n")
+	return strings.Join(output, "\n")
 }
 
 func groupBenchmarksByName(benchmarks []gtr.Benchmark) []gtr.Benchmark {