Removing `postinstall_message_id` from postinstall INSERT queries

Currently, the postinstall messages are inserted into the table with a `postinstall_message_id`.
This PR removes the `postinstall_message_id` from those INSERT queries and lets the database use the autoincrement value instead.

The `postinstall_message_id` isn't used as a foreign key in other tables, thus we don't have to specify it in the query.

Assuming that 3rd party extensions may use this messages as well, we will face problems if we want to insert a message in Joomla 3.2.x after a 3rd party already inserted some own messages. The id may already be taken then and our query will fail.

Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32330