CI: Don't install bundler

Author: mrkn

.github/workflows/ubuntu.yml

@@ -49,8 +49,6 @@ jobs:
         sudo pip3 install wheel
         sudo pip3 install -r ci/requirements.txt
 
-    - run: gem install bundler
-
     - run: bundle install --jobs 4 --retry 3
 
     - name: Run tests

.github/workflows/windows.yml

@@ -46,8 +46,6 @@ jobs:
 
     - run: pip3 install -r ci/requirements.txt
 
-    - run: gem install bundler
-
     - run: bundle install --jobs 4 --retry 3
 
     - name: Run tests