Merge #11391: Remove lxcbr0 lines from gitian-build.sh

Merge #11391: Remove lxcbr0 lines from gitian-build.sh

884913041 Remove lxcbr0 lines from gitian-build.sh (MeshCollider)

Pull request description:

  The `gitian-build.sh` script crashes with an error when I tried to use it, @kallewoof also had this same issue:

  	lxcbr0: ERROR while getting interface flags: No such device
  	SIOCSIFADDR: No such device
  	lxcbr0: ERROR while getting interface flags: No such device

  And then:

  	lxc-execute: failed to find gateway addresses
  	lxc-execute: failed to spawn 'gitian'
  	./bin/gbuild:21:in `system!': failed to run make-clean-vm --suite trusty --arch amd64 (RuntimeError)

  I believe it's because of the two lines which this PR removes, I tested it and seems to work as expected now. These lines are unique to this script and aren't mentioned in `gitian-building.md` or `release-process.md`. We discussed it on IRC, @achow101 agrees removing these lines would probably fix it: https://botbot.me/freenode/bitcoin-core-dev/2017-09-19/?msg=91299782&page=2

  Has anyone successfully used this script as-is? Or does everyone else manually run the builds/write their own script like I have up til this point?

Tree-SHA512: 0cffd3df307ad107fb1d4bae45094fc5002b56c2fe64f03642d968659fdc62f586ba249dbc540b69058b276f2456962e5bc4d665cab21ef1f561735eb78afcc2