Remove methods for clearing radio buttons

Methods exist and are created to clear radio buttons. These methods do not appear to work as intended.

Given a page:

````html
<html>
  <body>
    <input type="radio" name="group" value="A">A<br>
    <input type="radio" name="group" value="B">B
  </body>
</html>
````

And a page object:

````ruby
class MyPage
  include PageObject

  radio(:my_radio, :value => 'B')
end
````

When using Selenium-WebDriver, clearing the radio button does not change the radio button's state:

````ruby
page = MyPage.new(browser)
p page.my_radio_selected?
#=> false
page.select_my_radio
p page.my_radio_selected?
#=> true
page.clear_my_radio
p page.my_radio_selected?
#=> true
````

Watir-Webdriver removed the Radio#clear button several versions ago. As a result, trying to clear the radio button for a page object generates an exception:

````ruby
page = MyPage.new(browser)
p page.my_radio_selected?
#=> false
page.select_my_radio
p page.my_radio_selected?
#=> true
page.clear_my_radio
p page.my_radio_selected?
#=> C:/Ruby193/lib/ruby/gems/1.9.1/gems/page-object-1.0.2/lib/page-object/platforms/watir_webdriver/page_object.rb:960:in `instance_eval': undefined method `clear' for #<Watir::Radio:0x3677aa8> (NoMethodError)
#=> 	from (eval):1:in `process_watir_call'
#=> 	from C:/Ruby193/lib/ruby/gems/1.9.1/gems/page-object-1.0.2/lib/page-object/platforms/watir_webdriver/page_object.rb:960:in `instance_eval'
#=> 	from C:/Ruby193/lib/ruby/gems/1.9.1/gems/page-object-1.0.2/lib/page-object/platforms/watir_webdriver/page_object.rb:960:in `process_watir_call'
#=> 	from C:/Ruby193/lib/ruby/gems/1.9.1/gems/page-object-1.0.2/lib/page-object/platforms/watir_webdriver/page_object.rb:396:in `clear_radio'
#=> 	from C:/Ruby193/lib/ruby/gems/1.9.1/gems/page-object-1.0.2/lib/page-object/accessors.rb:438:in `block in radio_button'
#=> 	from pageobject.rb:30:in `<main>'
````

Given that the methods do not work as intended and a user cannot clear a radio button (without some sort of Javascript), this pull request suggests removing the methods.