Remove redundant configuration bits

[ashthespy]

Following up from #429

• [x] iptables.rules isn't being used, and is being set directly by iptables.service
• [x] xbindkeysrc.service is redundant with the move to triggerhappy https://github.com/volumio/Build/pull/456
• [x] Remove mpd.conf in favour of mpd.conf.tmpl
• [x] Remove ./volumio/etc/fstab.x86, we create it on-the-fly in chrootconfig.sh
• [x] Remove ./volumio/etc/ata-modules.x86, in favour of list in x86.sh
• [x] Remove ./volumio/etc/default/grub as it's now fetched from platform-x86 repo.
• [x] Remove netplug relics -- volumio-network.service and volumio-network.d/netplug.conf

[ashthespy]

Round 1 should be ready to test and merge, will open a new PR with more clean-up soon :-)