Revert "[vm] Add type to _AsyncAwaitCompleter.start."

This reverts commit 9983424.

Reason for revert: Breaks vm/dart/causal_stacks/async_throws_stack_no_causal_test

Original change's description:
> [vm] Add type to _AsyncAwaitCompleter.start.
> 
> Without the type, the call to the the parameter f() was being handled
> as a dynamic InstanceCall (smiable), so we had a megamorphic call to one
> of the Closure_XXXX_call methods (there are many copies of this method
> for different signatures) which in turn calls the closure.
> 
> Now that the type is present, we do a direct ClosureCall to the target.
> 
> This gives a measurable improvement to an important internal benchmark.
> 
> Change-Id: Ia9429879f69257465a2537616d8f8595fb1eefa8
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138520
> Reviewed-by: Martin Kustermann <[email protected]>
> Commit-Queue: Samir Jindel <[email protected]>

[email protected],[email protected]

Change-Id: I2deeebc24d80a7f444f4f53c5c8959b747fd1b29
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138502
Reviewed-by: Samir Jindel <[email protected]>
Commit-Queue: Samir Jindel <[email protected]>

Author: sjindel-google

sdk/lib/_internal/vm/lib/async_patch.dart

@@ -43,7 +43,7 @@ class _AsyncAwaitCompleter<T> implements Completer<T> {
     }
   }
 
-  void start(void Function() f) {
+  void start(f) {
     f();
     isSync = true;
   }

sdk_nnbd/lib/_internal/vm/lib/async_patch.dart

@@ -41,7 +41,7 @@ class _AsyncAwaitCompleter<T> implements Completer<T> {
     }
   }
 
-  void start(void Function() f) {
+  void start(f) {
     f();
     isSync = true;
   }