Remove go:nocheckptr pragma in Go bindings

According to @mdempsky, we should be able to remove the pragma by following this advice:

> Nevermind, I see. It's because the struct is packed, so the value field ends up unaligned for a Go pointer.

> I assume these are always pointers into C memory? If so, I think you can change the code to use *uintptr instead of **C.uint8_t, and then you should be able to remove the nocheckptr pragma.

This is related to the changes from #3033