[api] Remove warning about ValueSerializer

This API has existed for two and a half years now, with no
API changes over the last year, and is widely used in production,
code so it makes sense to consider it stable.

Change-Id: I10e38c37fb8c13e22124ef0985f4b0bd8d4615fd
Reviewed-on: https://chromium-review.googlesource.com/c/1461999Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59576}

include/v8.h

@@ -2008,10 +2008,6 @@ class V8_EXPORT JSON {
 /**
  * Value serialization compatible with the HTML structured clone algorithm.
  * The format is backward-compatible (i.e. safe to store to disk).
- *
- * WARNING: This API is under development, and changes (including incompatible
- * changes to the API or wire format) may occur without notice until this
- * warning is removed.
  */
 class V8_EXPORT ValueSerializer {
  public:
@@ -2132,10 +2128,6 @@ class V8_EXPORT ValueSerializer {
 /**
  * Deserializes values from data written with ValueSerializer, or a compatible
  * implementation.
- *
- * WARNING: This API is under development, and changes (including incompatible
- * changes to the API or wire format) may occur without notice until this
- * warning is removed.
  */
 class V8_EXPORT ValueDeserializer {
  public: