Remove docker stack config commandline reference doc

`docker stack config` command does not exists anymore, removing it
from the command reference documentation.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

docs/reference/commandline/stack_config.md

@@ -1,37 +0,0 @@
----
-title: "stack config"
-description: "The stack config command description and usage"
-keywords: "stack, config"
-advisory: "experimental"
----
-
-<!-- This file is maintained within the docker/docker Github
-     repository at https://github.com/docker/docker/. Make all
-     pull requests against that repo. If you see this file in
-     another repository, consider it read-only there, as it will
-     periodically be overwritten by the definitive file. Pull
-     requests which include edits to this file in other repositories
-     will be rejected.
--->
-
-# stack config (experimental)
-
-```markdown
-Usage:  docker stack config [OPTIONS] STACK
-
-Print the stack configuration
-
-Options:
-      --file   string   Path to a Distributed Application Bundle file (Default: STACK.dab)
-      --help            Print usage
-```
-
-Displays the configuration of a stack.
-
-## Related information
-
-* [stack deploy](stack_deploy.md)
-* [stack rm](stack_rm.md)
-* [stack services](stack_services.md)
-* [stack ps](stack_ps.md)
-* [stack ls](stack_ls.md)