set/get speed functions should be removed

I just noticed, set_speed and get_speed functions just don't do anything: 
  * they don't have a corresponding command in Selenium::Remote::Commands
  * the JSON wire protocol specification does not mention anything about those 
  * looking at StackOverflow, it seems like those functions were [removed a while ago from Selenium](http://stackoverflow.com/questions/8583445/setspeed-in-selenium-webdriver-using-ruby/8606602#8606602)

So those functions should be removed since they are off no use.