Reverted AST changes for first-of-type

Functions can't be used in combination with axes, so I'll just need to fix the
position() function to work properly.

lib/oga/css/parser.y

@@ -515,11 +515,7 @@ end
   # @return [AST::Node]
   #
   def on_pseudo_class_first_of_type
-    return s(
-      :eq,
-      s(:call, 'count', s(:axis, 'preceding-sibling', s(:call, 'name'))),
-      s(:int, 0)
-    )
+    return s(:eq, s(:call, 'position'), s(:int, 1))
   end
 
   ##

spec/oga/css/parser/pseudo_classes/first_of_type_spec.rb

@@ -4,13 +4,13 @@ describe Oga::CSS::Parser do
   context ':first-of-type pseudo class' do
     example 'parse the :first-of-type pseudo class' do
       parse_css(':first-of-type').should == parse_xpath(
-        'descendant::*[count(preceding-sibling::name()) = 0]'
+        'descendant::*[position() = 1]'
       )
     end
 
     example 'parse the a:first-of-type pseudo class' do
       parse_css('a:first-of-type').should == parse_xpath(
-        'descendant::a[count(preceding-sibling::name()) = 0]'
+        'descendant::a[position() = 1]'
       )
     end
   end