doc/code: drop mentions of GOPATH/pkg directory

It's already half gone and later will be all gone.
It's not worth explaining in an introduction doc.

Fixes #24506
Updates #4719

Change-Id: Ie48128b3aa090d84e0e734aa45f14a4480292913
Reviewed-on: https://go-review.googlesource.com/129679


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>