Drop the separate JRuby build step

release-gem builds the gem again with `rake release`, so building it beforehand did nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: hsbt

.github/workflows/push_gem.yml

@@ -41,10 +41,6 @@ jobs:
       - name: Install dependencies
         run: bundle install --jobs 4 --retry 3
 
-      - name: Build gem for JRuby
-        run: rake build
-        if: matrix.ruby == 'jruby'
-
       - name: Publish to RubyGems
         uses: rubygems/release-gem@7f9650160c1a4e7989fdc9855807bdbd421d8b6b # v1.4.1