Remove unused method NotificationEntry#canViewBeDismissed()

We forgot to remove this method while inlining a dismissibility refactor
flag. This is an example for querying view states through the
NotificationEntry, which we'd like to avoid.

Test: build systemui
Flag: EXEMPT refactor
Change-Id: Ib3c95bd925c82361b33806f5c9c22e40388699b6