Remove support for fallback proc on last scope

This was introduced but never actually used. The introduction of environments.last as the fallback for unfound variables means this functionality is already broken

test/integration/variable_test.rb

@@ -80,10 +80,7 @@ class VariableTest < Minitest::Test
     assigns['test'] = 'Tobi'
     assert_equal 'Hello Tobi', template.render!(assigns)
     assigns.delete('test')
-    e = assert_raises(RuntimeError) do
-      template.render!(assigns)
-    end
-    assert_equal "Unknown variable 'test'", e.message
+    assert_equal "Hello ", template.render!(assigns)
   end
 
   def test_multiline_variable