Drop unused coud backup dir constant (home-assistant#6172)

The constant PATH_CLOUD_BACKUP is not used anywhere in the codebase.
Remove it to clean up the code. This is a leftover from a removed
initial cloud backup support implementation and got missed in home-assistant#5464.

Author: agners

supervisor/docker/const.py

@@ -165,7 +165,6 @@ def from_status(cls, status: str) -> PullImageLayerStage | None:
 PATH_BACKUP = PurePath("/backup")
 PATH_SHARE = PurePath("/share")
 PATH_MEDIA = PurePath("/media")
-PATH_CLOUD_BACKUP = PurePath("/cloud_backup")
 
 # https://hub.docker.com/_/docker
 ADDON_BUILDER_IMAGE = "docker.io/library/docker"