fix _exists

acme.sh

@@ -252,7 +252,7 @@ _exists() {
     eval type "$cmd" >/dev/null 2>&1
   elif command >/dev/null 2>&1; then
     command -v "$cmd" >/dev/null 2>&1
-  elif which which >/dev/null 2>&1; then
+  else
     which "$cmd" >/dev/null 2>&1
   fi
   ret="$?"