Merge #187: musig-spec: remove it from this repo

cc07b8f musig-spec: remove it (Jonas Nick)

Pull request description:

  Moved to https://github.com/jonasnick/bips/blob/musig2/bip-musig2.mediawiki.

ACKs for top commit:
  real-or-random:
    ACK cc07b8f

Tree-SHA512: 67aebe6afbacd83153c465fcea794d36f07d067e21f767d9f82d7429458d91fe1df8a7289c10d9fa5b5458b1b6603b51a3349528dc8af6b0293f34f0b25c311f

Author: real-or-random