archive/tar, archive/zip: revert documentation of ErrInsecurePath

archive/tar, archive/zip: revert documentation of ErrInsecurePath

CL 452616 disables path security checks by default, enabling them
only when GODEBUG=tarinsecurepath=0 or GODEBUG=zipinsecurepath=0
is set. Remove now-obsolete documenation of the path checks.

For #55356

Change-Id: I4ae57534efe9e27368d5e67773a502dd0e56eff4
Reviewed-on: https://go-review.googlesource.com/c/go/+/458875
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>