cmd/doc: don't drop const/var block if first entry is unexported

cmd/doc: don't drop const/var block if first entry is unexported

The code assumed that if the first entry was unexported, all the
entries were. The fix is simple: delete a bunch of code.

Fixes #12286.

Change-Id: Icb09274e99ce97df4d8bddbe59d17a5c0622e4c6
Reviewed-on: https://go-review.googlesource.com/14780
Reviewed-by: Andrew Gerrand <adg@golang.org>