runtime: remove the asyncScheduler constant

There is no reason to specialize this per chip as it is only ever used
for JavaScript. Not only that, it is causing confusion and is yet
another quirk to learn when porting the runtime to a new
microcontroller.

src/runtime/runtime_wasm_js.go

@@ -40,8 +40,6 @@ func go_scheduler() {
 	scheduler()
 }
 
-const asyncScheduler = true
-
 func ticksToNanoseconds(ticks timeUnit) int64 {
 	// The JavaScript API works in float64 milliseconds, so convert to
 	// nanoseconds first before converting to a timeUnit (which is a float64),