Fix uninstall module  - layout_module issue.

Steps to replicate:

1. Log in to admin
2. Install a module like Account from Modules
3. Put the module on a layout like Home from Layouts
4. Go back to Modules and uninstall module Account

Expected results: the row from `layout_module` table for module Account and Layout Home must be deleted

Actual results: the row is not deleted and if you go now to Layout Home you will see other module replacing the uninstalled Account; if you will click save than this module will replace module Account in the DB table

This is a trivial issue and can create small problems when you will have many modules on a layout.