doc: remove Go Security Policy page (moved to x/website)

This page has moved to the x/website repo in CL 211300 (commit
golang/website@3c8b7f9).
Remove the old copy in this repo since it's no longer used.

Updates #29206

Change-Id: I8b3396d9e42d1e7262a8cde9577962d33b215836
Reviewed-on: https://go-review.googlesource.com/c/go/+/211301
Reviewed-by: Filippo Valsorda <[email protected]>

Author: dmitshur