Remove unused code and test

This test was failing in JRuby, and since it's not being used anywhere
in Haml any more, it's safe to just remove it.

lib/haml/util.rb

@@ -302,14 +302,6 @@ module Haml
       obj
     end
 
-    # Throws a NotImplementedError for an abstract method.
-    #
-    # @param obj [Object] `self`
-    # @raise [NotImplementedError]
-    def abstract(obj)
-      raise NotImplementedError.new("#{obj.class} must implement ##{caller_info[2]}")
-    end
-
     # Silence all output to STDERR within a block.
     #
     # @yield A block in which no output will be printed to STDERR

test/util_test.rb

@@ -260,17 +260,6 @@ class UtilTest < Test::Unit::TestCase
     assert_equal([:before, :around_before, :loaded], obj2.arr)
   end
 
-  class FooBar
-    def foo
-      Haml::Util.abstract(self)
-    end
-  end
-
-  def test_abstract
-    assert_raise_message(NotImplementedError,
-      "UtilTest::FooBar must implement #foo") {FooBar.new.foo}
-  end
-
   def test_def_static_method
     klass = Class.new
     def_static_method(klass, :static_method, [:arg1, :arg2],