Remove now unnecessary squash_instance_assigns_with_environments

Since it was just there to make sure instance assigns were overriden
when rending more than once with the same template instance.  It is
no longer necessary because the template no longer stores instance_assigns
which need to be overriden.

test/integration/template_test.rb

@@ -52,10 +52,7 @@ class TemplateTest < Minitest::Test
     t = Template.new
     assert_equal('from custom assigns', t.parse("{{ foo }}").render!('foo' => 'from custom assigns'))
     assert_equal('', t.parse("{{ foo }}").render!)
-  end
 
-  def test_custom_assigns_squash_instance_assigns
-    t = Template.new
     assert_equal('from instance assigns', t.parse("{% assign foo = 'from instance assigns' %}{{ foo }}").render!)
     assert_equal('from custom assigns', t.parse("{{ foo }}").render!('foo' => 'from custom assigns'))
   end