[gtk+] Remove more discontinued ChangeLogs

[William Jon McCann <mccann src gnome org>]

commit 2b571f41e2cfc4230871624c1a784bb112d85538
Author: William Jon McCann <william jon mccann gmail com>
Date:   Mon Jun 24 18:00:12 2013 -0400

    Remove more discontinued ChangeLogs
    
    They are all in git.

 docs/reference/ChangeLog         | 6527 -----------------------------
 gtk/a11y/ChangeLog               |  348 --
 modules/engines/pixbuf/ChangeLog |  418 --
 po-properties/ChangeLog          | 4018 ------------------
 po/ChangeLog                     | 8398 --------------------------------------
 5 files changed, 0 insertions(+), 19709 deletions(-)