spec: remove reference to Go 1.17 spec

Also, delete go1.17_spec.html.

Change-Id: I7c78029dcfbbe8dbabb4ca81052976c1c8f4ed9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/645717
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
TryBot-Bypass: Robert Griesemer <[email protected]>

Author: griesemer