hooks: uninstall Entire's hook from pre-commit's .legacy copy

In pre-commit's migration mode, Entire's hook lives at <hook>.legacy, which pre-commit's wrapper runs. Uninstall only looked at <hook>, saw the wrapper as foreign, and left Entire's hook in .legacy and the user's original in .pre-entire. Now it removes Entire's hook there and restores the backup into .legacy, so pre-commit runs what it would have without Entire.