remove entire 'default' attribute acccessor override method

This reverts us to the state before
725a17c, making OptRegexp simply
inherit from OptBase again.

Author: Brent Cook

lib/msf/core/option_container.rb

@@ -527,10 +527,6 @@ def normalize(value)
     return Regexp.compile(value)
   end
 
-  def default
-    @default
-  end
-
   def display_value(value)
     if value.kind_of?(Regexp)
       return value.source