[object] Update SlicedStrings' comment regarding ExternalStrings

See set_parent
https://source.chromium.org/chromium/chromium/src/+/main:v8/src/objects/string-inl.h;l=857;drc=a6207b54d010a5bee8611a6ef08f9bee9a9768cf

Change-Id: I2fa5718c04b281016e2d4cae07b7f36e37176d6e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2947402Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75023}

src/objects/string.h

@@ -791,8 +791,6 @@ class ThinString : public TorqueGeneratedThinString<ThinString, String> {
 // are not nested since the double indirection is simplified when creating
 // such a substring.
 // Currently missing features are:
-//  - handling externalized parent strings
-//  - external strings as parent
 //  - truncating sliced string to enable otherwise unneeded parent to be GC'ed.
 class SlicedString : public TorqueGeneratedSlicedString<SlicedString, String> {
  public: