directory: remove unused updateMp3Directory() function

It hasn't been used in many years

  commit 3a89afdd80
  Author: Warren Dukes <warren.dukes@gmail.com>
  Date:   Sat Nov 20 20:28:32 2004 +0000

      remove --update-db option

      (SVN r2719)

src/directory.h

@@ -60,8 +60,6 @@ int writeDirectoryDB(void);
 
 int readDirectoryDB(void);
 
-void updateMp3Directory(void);
-
 Song *getSongFromDB(const char *file);
 
 time_t getDbModTime(void);