Update docs and completion-scripts for deprecated features

- the `--disable-legacy-registry` daemon flag was removed
- duplicate keys with conflicting values for engine labels
  now produce an error instead of a warning.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

man/dockerd.8.md

@@ -26,7 +26,6 @@ dockerd - Enable daemon mode
 [**--default-ipc-mode**=*MODE*]
 [**--default-shm-size**[=*64MiB*]]
 [**--default-ulimit**[=*[]*]]
-[**--disable-legacy-registry**]
 [**--dns**[=*[]*]]
 [**--dns-opt**[=*[]*]]
 [**--dns-search**[=*[]*]]
@@ -197,9 +196,6 @@ $ sudo dockerd --add-runtime runc=runc --add-runtime custom=/usr/local/bin/my-ru
 **--default-ulimit**=[]
   Default ulimits for containers.
 
-**--disable-legacy-registry**=*true*|*false*
-  Disable contacting legacy registries. Default is `true`.
-
 **--dns**=""
   Force Docker to use specific DNS servers