Update admin.php

Do not unset all sessions. Unset the [security token](https://github.com/Awilum/morfy-cms/blob/master/libraries/Morfy/Morfy.php#L71) session only. https://github.com/tovic/comments-plugin-for-morfy-cms/commit/01eb8322296c673d8947041d5a8d92fef244a03a#diff-7f0d08392e832c76db6959b6dc455c56R239