_content/doc: revert "temporarily add Go language specification documents"

Revert commit 2e3ad7d (CL 393115).
By now, the website is using Go content from release-branch.go1.18,
so this temporary mitigation is no longer needed.

Fixes golang/go#51686.

Change-Id: I47e3962cdffd88c708e1c150d9cf4bdcac217b24
Reviewed-on: https://go-review.googlesource.com/c/website/+/393355
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Trust: Dmitri Shuralyov <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>

Author: dmitshur