hostapd: Remove obsolete dump_file variable

With dump_file assigned in hostapd.conf, hostapd prints out a warning
that the variable is deprecated:

   DEPRECATED: 'dump_file' configuration variable is not used anymore

Remove the assignment of dump_file in wandboard_hostapd.conf
init_ifupdown: remove redundant start/stop of hostapd

hostapd is started using init scripts after networking, so starting
hostapd in the network interface startup is redundant and causes
some warning messages.