unset some variables we no longer need

These are only used when z is sourced, so it's safe to get rid of them.

z.sh

@@ -218,3 +218,6 @@ elif complete &> /dev/null; then
   }
  }
 fi
+
+# we can unset these as they are only used during setup
+unset -v _Z_NO_PROMPT_COMMAND _Z_NO_RESOLVE_SYMLINKS _Z_RESOLVE_SYMLINKS