doc: remove code.html

This is being moved to x/website in https://golang.org/cl/199058.

Updates golang/go#33637
Updates golang/go#29206

Change-Id: I5c58b784fcdd212d7a003cd0f4085059f33a47c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/199057Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>