recovery_ui: Remove RecoveryUI::last_key.

It's a private member, and the last user has been removed in [1] in
2015.

[1] commit ec28340c,
https://android-review.googlesource.com/c/platform/bootable/recovery/+/146330

Test: mmma -j bootable/recovery
Change-Id: I65a2370cb20a7b296888425a44a42c8b90abc766