Remove changelog entry for part-of in 1.23.

Review-Url: https://codereview.chromium.org/2827433007 .

Author: floitschG

CHANGELOG.md

@@ -20,14 +20,6 @@
 
 ## 1.23.0
 
-### Language
-* Allow using URI strings in `part of` declarations to refer to the
-  importing library.
-  A library part now can declare its library either as:
-  `part of name.of.library;` or as `part of "uriReferenceOfLibrary.dart";`.
-  This allows libraries with no library declarations (and therefore no name)
-  to have parts, and it allows tools to easily find the library of a part file.
-
 #### Strong Mode
 
 * Breaking change - it is now a strong mode error if a mixin causes a name