Do not define SocketError as Ruby 1.8 is not longer supported

Author: abotalov

rb/lib/selenium/webdriver/firefox.rb

@@ -28,10 +28,3 @@ def self.path=(path)
     end
   end
 end
-
-
-# SocketError was added in Ruby 1.8.7.
-# If it's not defined, we add it here so it can be used in rescues.
-unless defined? SocketError
-  class SocketError < IOError; end
-end