Remove a missing dependency in MANIFEST.MF of hibernate-search

* This is really a required dependency.  However, the
    `org.hibernate.search.engine:main` does not exist yet in JBoss AS7 that
    ships with TorqueBox-2.3.0.  And, mentioning inexistent dependencies
    causes the deployment to fail. So, when such module becomes available
    we'll restore the dependency statement.