Simplify implementation and fix a test

Author: DougGregor

stdlib/public/Concurrency/MainActor.swift

@@ -67,10 +67,6 @@ extension MainActor {
     resultType: T.Type = T.self,
     body: @MainActor @Sendable () throws -> T
   ) async rethrows -> T {
-    @MainActor func runOnMain(body: @MainActor @Sendable () throws -> T) rethrows -> T {
-      return try body()
-    }
-
-    return try await runOnMain(body: body)
+    return try await body()
   }
 }

test/api-digester/stability-concurrency-abi.test

@@ -50,4 +50,5 @@
 // UNSUPPORTED: swift_evolve
 
 // *** DO NOT DISABLE OR XFAIL THIS TEST. *** (See comment above.)
+Func MainActor.run(resultType:body:) has generic signature change from <T where T : Swift.Sendable> to <T>
 // *** DO NOT DISABLE OR XFAIL THIS TEST. *** (See comment above.)