misc/sortac: delete sortac command

The sortac command is no longer needed as of CL 157238, and
can be deleted. Its functionality has been directly integrated
into the new x/build/cmd/updatecontrib command. A previous version
of updatecontrib was the only user of sortac.

Updates #12042

Change-Id: If7442ebee11d05d095ff875a37eed3973c0fd9ca
Reviewed-on: https://go-review.googlesource.com/c/157257
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: dmitshur