What obsolete files can I remove from my iPhoto Library?
It is a bad idea to try to manually modify the folder structure inside the iPhoto library, as you seem to want to do.

The best way is to follow Apple's instructions for rebuilding the iPhoto library.

However, this is usually only used as a repair procedure if iPhoto is malfunctioning.

You can try this, but make sure you have a complete backup of your iPhoto library before you start the rebuilding procedure.

Also, realize that iPhoto has an internal Trash that is separate from Mac OS X's Trash. If you have deleted hundreds of photos over the years, and never emptied the iPhoto Trash, those photos will still be in there and taking up space in the file structure.