Setting the `VAGRANT_DEFAULT_PROVIDER` environment variable breaks the unit tests

To reproduce:

    export VAGRANT_DEFAULT_PROVIDER='vmware_fusion'
    rake

Results in the following broken test:

    Failures:
    
      1) Vagrant::Environment default provider should return virtualbox
         Failure/Error: instance.default_provider.should == :virtualbox
           expected: :virtualbox
                got: :vmware_fusion (using ==)
           Diff:
           @@ -1,2 +1,2 @@
           -:virtualbox
           +:vmware_fusion
         # ./test/unit/vagrant/environment_test.rb:229:in `block (3 levels) in <top (required)>'
    
    Finished in 5.16 seconds
    432 examples, 1 failure, 1 pending

I think we can just delete the test because there is another test that already tests the same thing:

    describe "default provider" do
      it "is virtualbox without any environmental variable" do
        with_temp_env("VAGRANT_DEFAULT_PROVIDER" => nil) do
          subject.default_provider.should == :virtualbox
        end
      end
    
      it "is whatever the environmental variable is if set" do
        with_temp_env("VAGRANT_DEFAULT_PROVIDER" => "foo") do
          subject.default_provider.should == :foo
        end
      end
    end