Remove checks for invalid handlers

[tanyakavrakova]

Remove conditions that check if handlers are valid handlers for an event.

validate_handlers/2 first checks if __handled_events__ function is defined in each of the handlers and then checks if all handlers handle the event.
However, at the time of checking if __handled_events__ function is defined, the handler module may not be loaded. This will cause the function validate_handlers/2 to return incorrect falsy result.

Since the checks that validate handlers happen at run-time, this does not provide much more value compared to just letting it crash. That is why validate_handlers/2 function is entirely removed from the code.