Fix syntax error in confirmation prompt of uninstall.sh

We need to use POSIX syntax. This file is no longer executed with zsh.

Fixes #4483.

tools/uninstall.sh

 read -r -p "Are you sure you want to remove Oh My Zsh? [y/N] " confirmation
-if ! [[ $confirmation =~ ^[yY]$ ]]
+if [ $confirmation = y ] && [ $confirmation = Y ]
 then
-    echo "Uninstall cancelled"
-    exit
+  echo "Uninstall cancelled"
+  exit
 fi
 
 echo "Removing ~/.oh-my-zsh"