Drop rb5-userdebug build target and use db845c-userdebug for RB5 am: 2505ddff85 am: 11bf1f25f3

Original change: https://android-review.googlesource.com/c/device/linaro/dragonboard/+/3018835

Change-Id: I61aafa3b88023f11a0a903b422a4df05eb3a4984
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>