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>
e46d22976a