WL#17344: Remove gui_plugin from mysql-shell packages

mysql-shell packages currently ship gui_plugin and migration_plugin. This was done because of the old MySQL Shell GUI offering, which is being superseded by MySQL Workbench 26.7

Starting with MySQL Shell 26.10, gui_plugin and migration_plugin should no longer be distributed as part of the core, command-line MySQL Shell packages.

This is a configuration change, shell-plugins builds should not package gui and migration plugins, while Shell builds should not expect these plugins to exist.