Remove redundant test that breaks if `VAGRANT_DEFAULT_PROVIDER` is set.

test/unit/vagrant/environment_test.rb

@@ -224,12 +224,6 @@ describe Vagrant::Environment do
     end
   end
 
-  describe "default provider" do
-    it "should return virtualbox" do
-      instance.default_provider.should == :virtualbox
-    end
-  end
-
   describe "copying the private SSH key" do
     it "copies the SSH key into the home directory" do
       env = isolated_environment