Merge #20735: script: Remove outdated extract-osx-sdk.sh

1fbb8b6 script: Remove outdated extract-osx-sdk.sh (Hennadii Stepanov)

Pull request description:

  The removed script was used to prepare `MacOSX10.11.sdk` which we do not use since 0.20 (bitcoin/bitcoin@7e21044 from #16392).

  See bitcoin-core/docs#68.

ACKs for top commit:
  laanwj:
    ACK 1fbb8b6

Tree-SHA512: ab5128ec2c505a43b1a12e06abf38d8fa8f62a03efe760de2c842550e82f68ca512dee1b6c220a4f8af2465a9c35a429ca91cb179cc4ce016fcd87c61fbc8b07

Author: laanwj