Remove the deprecated ValueSerializer::ReadHeader overload with no context parameter.

Several weeks have now passed since it was deprecated; Chromium does not
call it.

BUG=chromium:148757

Review-Url: https://codereview.chromium.org/2478863002
Cr-Commit-Position: refs/heads/master@{#40945}

include/v8.h

@@ -1819,7 +1819,6 @@ class V8_EXPORT ValueDeserializer {
    * May, for example, reject an invalid or unsupported wire format.
    */
   V8_WARN_UNUSED_RESULT Maybe<bool> ReadHeader(Local<Context> context);
-  V8_DEPRECATE_SOON("Use Local<Context> version", Maybe<bool> ReadHeader());
 
   /*
    * Deserializes a JavaScript value from the buffer.

src/api.cc

@@ -3115,11 +3115,6 @@ Maybe<bool> ValueDeserializer::ReadHeader(Local<Context> context) {
   return Just(true);
 }
 
-Maybe<bool> ValueDeserializer::ReadHeader() {
-  Isolate* isolate = reinterpret_cast<Isolate*>(private_->isolate);
-  return ReadHeader(isolate->GetEnteredContext());
-}
-
 void ValueDeserializer::SetSupportsLegacyWireFormat(
     bool supports_legacy_wire_format) {
   private_->supports_legacy_wire_format = supports_legacy_wire_format;