IS_JOYCONFIG was removed back in 2015 (1b7576aa), but there are still some mentions of retroarch-joyconfig in the docs and Makefiles. Clean these up so folk aren't confused about the fact it's no longer available.
1b7576aa
- Replaced crap A7A7 make - Added better targets and clean targets for Sega Mega Drive Mini and (S)NES Classic