Plugins: Remove obsolete `plugin_slugs` transient from `WP_Plugins_List_Table::prepare_items()`.

The transient is only used in the `wp_dashboard_plugins_output()` function, which is deprecated and unused since [40607].

The function itself also sets the same transient, so it can be removed from `WP_Plugins_List_Table`.

Props aftabmuni.
Fixes #49340.
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git-svn-id: https://core.svn.wordpress.org/trunk@46967 1a063a9b-81f0-0310-95a4-ce76da25c4cd