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/usr/libexec/docker-failure

@@ -1,11 +0,0 @@
-#!/bin/sh
-
-if [ "$SERVICE_RESULT" = "exit-code" ] && [ "$EXIT_STATUS" = "1" ]; then
-    echo "Docker exited with exit status 1, this might be caused by corrupted key.json."
-    size=$(stat -c %s "/mnt/overlay/etc/docker/key.json")
-    echo "key.json: ${size} bytes"
-    if ! jq -e < "/mnt/overlay/etc/docker/key.json" > /dev/null || [ "${size}" -eq 0 ]; then
-        echo "key.json appears to be corrupted, it is not parsable. Removing it."
-        rm -f "/mnt/overlay/etc/docker/key.json"
-    fi
-fi