Remove stale entries from Unreleased section. (#4185)

Author: rlazo

firebase-perf/CHANGELOG.md

@@ -1,10 +1,5 @@
 # Unreleased
 
-* [fixed] Fixed a bug where screen traces were not capturing frame metrics for multi-activity apps.
-* [feature] Added support for measuring screen performance metrics for "Activity Fragments" out-of-the-box.
-* [fixed] Excluded custom attributes whose key/value length is 0.
-
-
 # 20.2.0
 * [unchanged] Updated to accommodate the release of the updated
   [perfmon] Kotlin extensions library.