doc: remove The Go Project page (moved to x/website)

This page has moved to the x/website repo in CL 229482 (commit
golang/website@70f4ee8).
Remove the old copy in this repo since it's no longer used.

For golang#29206.

Change-Id: Ief093ed8c5dfec43e06d473e4282275f61da74a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/229485
Reviewed-by: Alexander Rakoczy <[email protected]>