d5e1615 Remove gitian-building docs (MarcoFalke)

Pull request description:

  The docs have been archived in commit 38befdf: https://github.com/bitcoin-core/docs/blob/38befdf71ba21e69a598e974f7c06a9cdb5b0dfd/gitian-building.md

  Fixes #76

ACKs for top commit:
  fanquake:
    ACK d5e1615

Tree-SHA512: efc1a50361e1b166b1a6f13150e06ff47106244e19c25c234caf98c5db1312c0b752655cb60aac18528c14ee898ac3bed7ecc00a341f355d0b33009cdf0c91ee