Remove win32 from Github release

Summary: We no longer generate win32 binaries.

Test Plan:

./contrib/release/github-release.sh -a "/binaries/0.21.4/" -o /path/to/github-oauth-token -t v0.21.4 -r doc/release-notes/release-notes-0.21.4.md --dry-run

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5721