fix(demo): remove unused go/ast import from gotypes demo

Removed the unused "go/ast" import from _demo/go/gotypes/main.go
that was causing compilation errors. The demo now compiles and runs
successfully with both standard go and llgo.

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: luoliwoshang <51194195+luoliwoshang@users.noreply.github.com>

_demo/go/gotypes/main.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"fmt"
-	"go/ast"
 	"go/token"
 	"go/types"
 )