docs(config): drop legacy mount_file option

All logic that used this was completely removed as of #30991

Signed-off-by: Josh <josh.t.richards@gmail.com>

Author: joshtrichards

config/config.sample.php

@@ -2482,13 +2482,6 @@
 	 */
 	'part_file_in_storage' => true,
 
-	/**
-	 * Specify the location of the ``mount.json`` file.
-	 *
-	 * Defaults to ``data/mount.json`` in the Nextcloud directory.
-	 */
-	'mount_file' => '/var/www/nextcloud/data/mount.json',
-
 	/**
 	 * Prevent Nextcloud from updating the cache due to filesystem changes for all
 	 * storage.