Get rid of #define byte in PlatformDefs.h

As far as I could tell (OSX) used only in rarlib + iso9660.  Removed in the latter, and it was already typedef'd in the former.

IOS, Android, OSX built fine.  Needs linux (don't anticipate any issues) and win32 (jenkins build failed).

Reverts the kludgy fix of #2727, thus needs testing on rpi.