plugins: hide deprecated command

src/poetry/console/commands/plugin/add.py

@@ -43,6 +43,7 @@ It works similarly to the <c1>add</c1> command:
 
 {deprecation}
 """
+    hidden = True
 
     def handle(self) -> int:
         self.line_error(self.deprecation)

src/poetry/console/commands/plugin/remove.py

@@ -36,6 +36,8 @@ class PluginRemoveCommand(Command):
         "</warning>"
     )
 
+    hidden = True
+
     def handle(self) -> int:
         self.line_error(self.help)
 

src/poetry/console/commands/plugin/show.py

@@ -19,6 +19,7 @@ class PluginShowCommand(Command):
         "<warning>This command is deprecated. Use <c2>self show plugins</> "
         "command instead.</warning>"
     )
+    hidden = True
 
     def handle(self) -> int:
         self.line_error(self.help)