Deprecate LFS_F_OPENED and use lfs_mlist_isused instead

Instead of additional flag, we can just go through the mlist.

lfs.h

@@ -143,7 +143,6 @@ enum lfs_open_flags {
     LFS_F_ERRED   = 0x080000, // An error occurred during write
 #endif
     LFS_F_INLINE  = 0x100000, // Currently inlined in directory entry
-    LFS_F_OPENED  = 0x200000, // File has been opened
 };
 
 // File seek flags