package: remove unneeded 'npm prune'

prune just removes packages in node_modules that are not in
package.json, which is not necessary since we just removed node_modules

bin/release-_pre.sh

@@ -6,6 +6,5 @@ npm run update-authors
 git diff --exit-code
 rm -rf node_modules/
 npm install
-npm prune
 npm dedupe
 npm test