Revert "Add plugin for native autocompletion from Symfony 6 (#11999)" (#12493)

This reverts commit 166b9dcfdc.

plugins/symfony6/README.md

@@ -1,9 +0,0 @@
-# Symfony
-
-This plugin provides native completion for [Symfony](https://symfony.com/), but requires at least Symfony 6.2.
-
-To use it add `symfony6` to the plugins array in your zshrc file.
-
-```bash
-plugins=(... symfony6)
-```