remove redundant grammar rule

Author: adqm

Grammar/python.gram

@@ -718,7 +718,6 @@ expression[expr_ty] (memo):
 
 if_expression[expr_ty]:
     | a=disjunction 'if' b=disjunction 'else' c=expression { _PyAST_IfExp(b, a, c, EXTRA) }
-    | invalid_if_expression
 
 yield_expr[expr_ty]:
     | 'yield' 'from' a=expression { _PyAST_YieldFrom(a, EXTRA) }