Remove PersistentContainerCallbackType::kWeak

From what I understand, it's been slated for removal for a long time, and
more accutely, Clang recently started warning about it being shadowed by
another kWeak enum:

../../v8/include/v8-profiler.h:369:5: warning: declaration shadows a variable
in namespace 'v8' [-Wshadow]
    kWeak = 6              // A weak reference (ignored by the GC).
    ^
../../v8/include/v8-util.h:29:3: note: previous declaration is here
  kWeak = kWeakWithParameter  // For backwards compatibility.  Deprecate.
  ^

Bug: chromium:895475
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I07de1418aea2d68ef5641ece4ab13f71b6d5ec7f
Reviewed-on: https://chromium-review.googlesource.com/c/1285189Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56743}

include/v8-util.h

@@ -25,8 +25,7 @@ enum PersistentContainerCallbackType {
   kNotWeak,
   // These correspond to v8::WeakCallbackType
   kWeakWithParameter,
-  kWeakWithInternalFields,
-  kWeak = kWeakWithParameter  // For backwards compatibility.  Deprecate.
+  kWeakWithInternalFields
 };
 
 /**