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

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

Change-Id: I0fd4641bc89b93934a0ba20db856c734860e1e7f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>