Drop protect-main.sh: main's protection is the maintainer's, by hand #68

The maintainer's instruction (2026-10-03): no script makes or changes main's protection; the maintainer manages it in the repository settings and keeps main locked, and merges the release PR. Done when .github/scripts/release/protect-main.sh is deleted and the release README and PROJECT.md say so. python-multiplatform did the same in its PR #127.