MDL-22052 Reverted - string_exists uses caches again

I added the third parameter in the previous commit without exploring the
consequences. Now I see that string_exists is called too many often and
it must not rebuild caches again and again.

lib/moodlelib.php

@@ -5914,7 +5914,7 @@ class core_string_manager implements string_manager {
             return false;
         }
         $lang = current_language();
-        $string = $this->load_component_strings($component, $lang, true);
+        $string = $this->load_component_strings($component, $lang);
         return isset($string[$identifier]);
     }