Benchmarks: Remove DispatchSemaphore hack from Monoids benchmark

benchmark/multi-source/Monoids/Benchmark.swift

@@ -1,5 +1,4 @@
 import TestsUtils
-import Dispatch
 
 public let benchmarks = [
   BenchmarkInfo(
@@ -8,13 +7,8 @@ public let benchmarks = [
     tags: [.algorithm, .miniapplication, .long])
 ]
 
-func run_Monoids(_ n: Int) {
-  let semaphore = DispatchSemaphore(value: 0)
+func run_Monoids(_ n: Int) async {
   for _ in 0 ... n {
-    Task {
-      await run(output: false)
-      semaphore.signal()
-    }
-    semaphore.wait()
+    await run(output: false)
   }
 }