cli/command/plugin: remove DCT

Plugins are not widely used, and there's no known plugins that use
content-trust. We're working on updating the authentication stack
in the CLI, and the trust implementation hinders us in making
changes, so removing parts that are not high-priority (ahead of
full deprecation of DCT).

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

docs/reference/commandline/plugin_push.md

@@ -3,12 +3,6 @@
 <!---MARKER_GEN_START-->
 Push a plugin to a registry
 
-### Options
-
-| Name                      | Type   | Default | Description        |
-|:--------------------------|:-------|:--------|:-------------------|
-| `--disable-content-trust` | `bool` | `true`  | Skip image signing |
-
 
 <!---MARKER_GEN_END-->