Removes unused lifetime extension flag

Notification lifetime extension refactor flag not used;
superseded by android.app.lifetimeExtensionRefactor flag
which can be used in systemui and system server.

Bug: 299448097
Test: Removing unused flag
Flag: ACONFIG android.app.lifetimeExtensionRefactor DEVELOPMENT
Change-Id: I7d66bb3745809993743b0b7cd0b9684bded6f5cb