Revert r21912 : "Storage API : remove undeeded target-specific functions"

After a discussion mixed on mailing list and irc, it was agreed that
more abstraction for plugins is better (so developers don't have to add
a check to HAVE_DISK_STORAGE when writing disk-specific code)

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@21923 a1c6a512-1295-4272-9138-f99709370657

apps/plugins/clock/clock_settings.c

@@ -173,9 +173,7 @@ void load_settings(void){
             draw_message(display, MESSAGE_ERRLOAD, 1);
         display->update();
     }
-#ifdef HAVE_DISK_STORAGE
     rb->storage_sleep();
-#endif
     rb->sleep(HZ);
 }