revert warning if z.sh is being exec'd and not sourced

Because we can't reliably determine if we're really being exec'd.

z.sh

@@ -22,11 +22,6 @@
 #   * z -l foo  # list matches instead of cd
 #   * z -c foo  # restrict matches to subdirs of $PWD
 
-case $- in
- *i*) ;;
-   *) echo 'ERROR: z.sh is meant to be sourced, not directly executed.'
-esac
-
 [ -d "${_Z_DATA:-$HOME/.z}" ] && {
     echo "ERROR: z.sh's datafile (${_Z_DATA:-$HOME/.z}) is a directory."
 }