Remove unused variables in shell scripts.

Summary:

Remove unused variables in shell scripts. Use _ where we don't care
about the result.

Backport of core PR10771 and PR11649
https://github.com/bitcoin/bitcoin/pull/10771/files
https://github.com/bitcoin/bitcoin/pull/11649/files

Depends on D3955.

Not much change remaining as most of the files have been removed.

Test Plan:
Run the git-subtree-check.sh script. It will not output GOOD as we
made changes to all the subtree libraries (at least code formatting).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3957