ci: drop needless bundler

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

.github/workflows/linux.yml

@@ -28,6 +28,6 @@ jobs:
       env:
         CI: true
       run: |
-        gem install bundler rake
+        gem install rake
         bundle install --jobs 4 --retry 3
         bundle exec rake test