[makeinstancesufo] should not wipe out lib.plist in generated instances

[punchcutter]

Currently in postProcessInstance we have clearCustomLibs which wipes out everything in lib.plist except for public.glyphOrder and public.postscriptNames. I noticed by trying to copy the lib info to instances by setting <lib copy="1" /> in the designspace file. We shouldn't be deleting information, especially if the user is explicitly trying to copy it in the designspace.