Remove `* Take StringPiece instead of const string& in absl stubs.` from CHANGES.txt since we're not taking it.

CHANGES.txt

diff --git a/CHANGES.txt b/CHANGES.txt
index c3124d7..e6593c7 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -3,7 +3,6 @@
   * Handle reflection for message splitting.
   * make metadata fields lazy.
   * Extend visibility of plugin library to upb
-  * Take StringPiece instead of const string& in absl stubs.
   * Modernize conformance_cpp.cc.
   * Don't request 64-byte alignment unless the toolchain supports it.