reflect: remove deprecation notices from SliceHeader, StringHeader

reflect: remove deprecation notices from SliceHeader, StringHeader

There has been no proposal discussion about adding these notices.
Also, even if we did decide to add them, then since their replacements
are only appearing in Go 1.20, go.dev/wiki/Deprecation says that we
should wait until Go 1.22 to add the deprecation notice.

Filed #56906 for the proposal discussion.

Fixes #56905.

Change-Id: If86cce65aa00b4b62b2b18e82503431dcbdbcfed
Reviewed-on: https://go-review.googlesource.com/c/go/+/452761
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>