bpo-36623: Clean parser headers and include files (GH-12253)

After the removal of pgen, multiple header and function prototypes that lack implementation or are unused are still lying around.
diff --git a/Parser/parsetok.c b/Parser/parsetok.c
index ba33a9a..31be0eb 100644
--- a/Parser/parsetok.c
+++ b/Parser/parsetok.c
@@ -1,7 +1,7 @@
 
 /* Parser-tokenizer link implementation */
 
-#include "pgenheaders.h"
+#include "Python.h"
 #include "tokenizer.h"
 #include "node.h"
 #include "grammar.h"