ci: remove npm update step (not needed for pnpm publish --provenance)

pnpm publish --provenance uses pnpm's own OIDC implementation
and does not require updating npm CLI separately.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: yamatatsu

.github/workflows/release.yml

@@ -31,8 +31,6 @@ jobs:
         with:
           version: 10
           run_install: false
-      - name: Update npm (required for OIDC trusted publishing)
-        run: npm install -g npm@latest
       - run: pnpm install
       - run: pnpm test run
       - run: pnpm build