[sysadmin-bin] Drop GitHub mirror post-receive hook

[Bartłomiej Piotrowski <bpiotrowski src gnome org>]

commit 0d7ff9aeb2b60f41ed95b6fd4979613b26f4e419
Author: Bartłomiej Piotrowski <bpiotrowski gnome org>
Date:   Fri Jul 17 08:44:31 2020 +0200

    Drop GitHub mirror post-receive hook

 git/gnome-post-receive         |   1 -
 git/post-receive-github        |   7 --
 git/post-receive-mirror-github | 249 -----------------------------------------
 3 files changed, 257 deletions(-)
---
diff --git a/git/gnome-post-receive b/git/gnome-post-receive
index 4e42943..c1ee339 100755
--- a/git/gnome-post-receive
+++ b/git/gnome-post-receive
@@ -36,5 +36,4 @@ tee >($BINDIR/log-push 1>&2) \
     >($BINDIR/post-receive-update-description 1>&2) \
     >($BINDIR/post-receive-notify-l10n 1>&2) \
     >($BINDIR/post-receive-notify-kgb 1>&2) \
-    >($BINDIR/post-receive-github 1>&2) \
     | $BINDIR/gnome-post-receive-email 1>&2