go/doc: remove unused top-level declarations from playable example

go/doc: remove unused top-level declarations from playable example

When we synthesize a playable example, prune declarations that may be
in the original example file but aren't used by the example.

This is ported from pkgsite, where it fixed #43658.

Change-Id: I41e6d4c28afa993c77c8a82b47bd86ba15ed13b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/401758
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>