Revert "Added onLoad method to notify subclasses that data has been loaded from the database, in order perform relevant updates."

This reverts commit 2be346ed4a12668ec452a5d2416a626b515ce14c.
Removed unused `database` field from SugarRecord

This also produced a NullPointerException upon call to the constructor if the SugarApp was not added in the manifest.

Now the NullPointerException is produced upon save of the object, which makes more sense in my opinion, although it would be preferable to have another type of exception being produced given that particular case.