Silence output of 'which' command used in smbclient detection

Opening the admin panel always causes these lines to appear in my apache log:

---
which: no smbclient in (/sbin:/usr/sbin:/bin:/usr/bin)
which: no smbclient in (/sbin:/usr/sbin:/bin:/usr/bin)
which: no smbclient in (/sbin:/usr/sbin:/bin:/usr/bin)
---

As the output to stderr is not used, it can just be dropped. Detection of the command still works as before.