Nuke plblockimp.

It was used for building Nu with OS X 10.6 support, but we're 10.8+ now.
Nuke MHSysTree files, since they are no longer being used.
Drop saved_symbols file; we can't use it in XCode as far as I can see.

We'll probably still want to do something like this at some point, but we'll worry about it later.
Added a bunch of missing stuff to the XCode project.
Start cleaning out Makefile.