Remove rubyforge_name from Rakefile.

Hi, when I try to run rake -T I get this error:

rake aborted!
NoMethodError: undefined method `rubyforge_name=' for #<Hoe:0x007f7d9424bf30>
/home/gramos/srcs/rubygems-mirror/Rakefile:18:in `block in <top (required)>'
/home/gramos/srcs/rubygems-mirror/Rakefile:6:in `<top (required)>'
(See full trace by running task with --trace)

This change fix that problem.