source3: remove sock_exec

source3: remove sock_exec

Remove the sock_exec code which is no longer needed and additionally has been
used by exploit code.

This was originally test support code, the tests relying on the sock_exec
code have been removed.

Past exploits have used sock_exec as a proxy for system() matching a talloc
destructor prototype.

See for example:
Exploit for Samba vulnerabilty (CVE-2015-0240) at
    https://gist.github.com/worawit/051e881fc94fe4a49295
    and the Red Hat post at
    https://access.redhat.com/blogs/766093/posts/1976553

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Mon Nov 20 07:20:13 CET 2017 on sn-devel-144