Remove test for Fiber.current being available without extra require

We don't require fiber anywhere.

core/fiber/current_spec.rb

@@ -1,10 +1,6 @@
 require_relative '../../spec_helper'
 
 describe "Fiber.current" do
-  it "is available without an extra require" do
-    ruby_exe("print Fiber.current.class", options: '--disable-gems --disable-did-you-mean').should == "Fiber"
-  end
-
   it "returns the root Fiber when called outside of a Fiber" do
     root = Fiber.current
     root.should be_an_instance_of(Fiber)