Remove handling of Docker key.json (#4361)

The deprecated-key-path option is no longer handled, but it doesn't cause
problems because the key is explicitly ignored. It was completely removed in
Docker 19.03.0 [1].

As such, the option and the pre-start script to fix the corrupted key.json can
be removed now, as it has no effect, only printing confusing message when
Docker service fails to start.

[1] 98fc09128b

buildroot-external/rootfs-overlay/etc/docker/daemon.json

@@ -4,6 +4,5 @@
         "tag": "{{.Name}}"
     },
     "data-root": "/mnt/data/docker",
-    "deprecated-key-path": "/mnt/overlay/etc/docker/key.json",
     "bip": "172.30.232.1/23"
 }