Docs: remove info on already removed experimental plugins

Commit f1bc75eb removed these plugins, but the documentation
placeholders were left over.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

docs/source/future/index.rst

@@ -15,7 +15,3 @@ Experimental Plugins
    :maxdepth: 1
 
    plugins/cli.cmd.nrun
-   plugins/cli.cmd.runnable-run-recipe
-   plugins/cli.cmd.runnable-run
-   plugins/cli.cmd.task-run
-   plugins/cli.cmd.task-run-recipe

docs/source/future/plugins/cli.cmd.runnable-run-recipe.rst

-cli.cmd.runnable-run-recipe
-===========================

docs/source/future/plugins/cli.cmd.runnable-run.rst

-cli.cmd.runnable-run
-====================
-
-runner: runs one runnable
-

docs/source/future/plugins/cli.cmd.task-run-recipe.rst

-cli.cmd.task-run-recipe
-=======================

docs/source/future/plugins/cli.cmd.task-run.rst

-cli.cmd.task-run
-================