Remove the Symfony 1.x documentation

Remove all the documentation related to the integration of Propel in a
Symfony 1.x project. The sfPropelORMPlugin won't be updated for the next
Propel major release (2.0).
Remove mentions about dbd2propel

The dbd2propel command has been removed. The whole cookbook on
DBDesigner is deprecated.
Update a little bit the doc for PHP 5.4