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