DTV: always override the demux (refs #11185)

...like other access plugins already do. The input thread does not care
if the original psz_demux value is overriden.

modules/access/dtv/access.c

@@ -479,11 +479,8 @@ static int Open (vlc_object_t *obj)
 
     access->pf_block = Read;
     access->pf_control = Control;
-    if (access->psz_demux == NULL || !access->psz_demux[0])
-    {
     free (access->psz_demux);
     access->psz_demux = strdup ("ts");
-    }
     return VLC_SUCCESS;
 
 error: