webadmin: Removed custom MAC leftovers from dialog

The code meant to remove the custom MAC address checkbox and
explanation label didn't work because it was called asynchronously,
and apparently would only get disabled after the widget would get
edited and thus have no effect.

Change-Id: I5fc1bb2183b87567213ac9c5e8cb83e8a728979a
Bug-Url: https://bugzilla.redhat.com/1007506
Signed-off-by: Lior Vernia <lvernia@redhat.com>