Remove page on event name check

This is safe to do so as we will no longer pass a rogue string to this
method. Only the page on event names can be passed to this function.

Unless, of course, somebody converts a string with an incorrect name to
PageOnEventName to call this function. This will easily be catched in
reviews, and nobody would do such a thing. We follow the Go idiom of
being pragmatic instead of being highly defensive. Also, this method is
used by the mapping layer, and there is already an event name check.

common/page.go

@@ -1113,13 +1113,6 @@ type PageOnEvent struct {
 // passing in the ConsoleMessage associated with the event.
 // The only accepted event value is 'console'.
 func (p *Page) On(event PageOnEventName, handler func(PageOnEvent)) error {
-	switch event {
-	case EventPageConsoleAPICalled:
-	case EventPageMetricCalled:
-	default:
-		return fmt.Errorf("unknown page event: %q", event)
-	}
-
 	p.eventHandlersMu.Lock()
 	defer p.eventHandlersMu.Unlock()