Delete updateNotificationChannelGroupFromPrivilegedListener

This is an unpublished (AIDL only) method and was never used from NLS (or anywhere else).

Bug: 426205822
Test: build
Flag: EXEMPT Code deletion
(cherry picked from commit 6077349a)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:6d846a1a76fd2a45cf4cd9575673db2644b153fc
Merged-In: I65438b1af637f3ba9b75a85f780bdb7867cc4ba8
Change-Id: I65438b1af637f3ba9b75a85f780bdb7867cc4ba8