The comment in ast_for_namedexpr shouldn't include if_stmt (GH-17586)

Automerge-Triggered-By: @gvanrossum (cherry picked from commit b08d3f7) Co-authored-by: Guido van Rossum <guido@python.org>

Author: miss-islington

Python/ast.c

@@ -1935,9 +1935,7 @@ ast_for_decorated(struct compiling *c, const node *n)
 static expr_ty
 ast_for_namedexpr(struct compiling *c, const node *n)
 {
- /* if_stmt: 'if' namedexpr_test ':' suite ('elif' namedexpr_test ':' suite)*
- ['else' ':' suite]
- namedexpr_test: test [':=' test]
+ /* namedexpr_test: test [':=' test]
  argument: ( test [comp_for] |
  test ':=' test |
  test '=' test |