doc: move Go 1.19 release notes to x/website

Now that the development of the Go 1.19 release is almost done, its
release notes are moved to their eventual long-term home in x/website
in CL 420417. Delete the initial development copy here.

For golang#51400.

Change-Id: I741285555af28ce9a64e7f8d2b9fe2a0f3e13c26
Reviewed-on: https://go-review.googlesource.com/c/go/+/420418
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>