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>

src/reflect/value.go

@@ -2743,7 +2743,7 @@ func (v Value) UnsafePointer() unsafe.Pointer {
 // it references will not be garbage collected, so programs must keep
 // a separate, correctly typed pointer to the underlying data.
 //
-// Deprecated: Use unsafe.String or unsafe.StringData instead.
+// In new code, use unsafe.String or unsafe.StringData instead.
 type StringHeader struct {
 	Data uintptr
 	Len  int
@@ -2756,7 +2756,7 @@ type StringHeader struct {
 // it references will not be garbage collected, so programs must keep
 // a separate, correctly typed pointer to the underlying data.
 //
-// Deprecated: Use unsafe.Slice or unsafe.SliceData instead.
+// In new code, use unsafe.Slice or unsafe.SliceData instead.
 type SliceHeader struct {
 	Data uintptr
 	Len  int