doc: remove Release History pages (moved to x/website)

These pages were moved to the x/website repo in CL 210797 (commit
golang/website@9aef1eefbbe663d448b04b7cc1b2b995f4cf4c0b).
Remove the old copies in this repo since they're no longer used.

Updates #36075
Updates #29206

Change-Id: I6e3ffaebd92fa753cb5f3b21e4238edfb7f5f0e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/210798Reviewed-by: Alexander Rakoczy <alex@golang.org>