Fix backwards logic in uninstaller once and for all

Really this time :P

Fixes #4533

tools/uninstall.sh

 read -r -p "Are you sure you want to remove Oh My Zsh? [y/N] " confirmation
-if [ $confirmation = y ] || [ $confirmation = Y ]
+if [ "$confirmation" != y ] && [ "$confirmation" != Y ]
 then
   echo "Uninstall cancelled"
   exit