We don't need to maintain Kotlin extensions anymore (b/149288919)

Author: ramanpreetSinghKhinda

docs/README.md

@@ -31,7 +31,6 @@ in your app:
   * [`firebase-functions`](ktx/functions.md)
   * [`firebase-inappmessaging`](ktx/inappmessaging.md)
   * [`firebase-inappmessaging-display`](ktx/inappmessaging-display.md)
-  * [`firebase-perf`](ktx/perf.md)
   * [`firebase-remote-config`](ktx/remote-config.md)
   * [`firebase-storage`](ktx/storage.md)
   * [`firebase-database`](ktx/database.md)