Revert "reflect: deprecate (Slice|String)Header"

This reverts CL 401434.

Reason for revert: We should't deprecate these without a replacement.

Fixes #53079

Change-Id: I5515b03e39787d4e528a98c733cdb643b86b17ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/408815
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>

Author: ianlancetaylor

doc/go1.19.html

@@ -299,11 +299,6 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 
 <dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
   <dd>
-    <p><!-- CL 401434 -->
-      <a href="/pkg/reflect/#SliceHeader"><code>SliceHeader</code></a>
-      and <a href="/pkg/reflect/#StringHeader"><code>StringHeader</code></a>
-      are now deprecated, and their use should be avoided.
-    </p>
     <p><!-- CL 357331 -->
       TODO: <a href="https://go.dev/cl/357331">https://go.dev/cl/357331</a>: allow Value.Bytes on addressable byte arrays
     </p>

src/reflect/value.go

@@ -2667,9 +2667,6 @@ func (v Value) UnsafePointer() unsafe.Pointer {
 // Moreover, the Data field is not sufficient to guarantee the data
 // it references will not be garbage collected, so programs must keep
 // a separate, correctly typed pointer to the underlying data.
-//
-// Deprecated: this type should not be used, it exists only for
-// backward compatibility.
 type StringHeader struct {
 	Data uintptr
 	Len  int
@@ -2681,9 +2678,6 @@ type StringHeader struct {
 // Moreover, the Data field is not sufficient to guarantee the data
 // it references will not be garbage collected, so programs must keep
 // a separate, correctly typed pointer to the underlying data.
-//
-// Deprecated: this type should not be used, it exists only for
-// backward compatibility.
 type SliceHeader struct {
 	Data uintptr
 	Len  int