Remove deleted user actions from configuration

In Settings/Configure Kid3.../User Actions, context menu commands can be deleted (right mouse button, Delete row) and they disappear from the table. However, when the configuration is stored, it is not checked if there are old commands which have to be deleted. If Kid3 is then restarted, the deleted commands will reappear. The total number of commands can only be increased, but not decreased.

This patch fixes this problem by deleting stale commands when writing the configuration.