Remove bundle install from docs-preview pipeline

[zzak]

The reason for bundle install was because the initial PR we used the ruby:latest docker image, which doesn't have any dependencies pre-installed.

As such, we needed BUNDLE_WITHOUT to prevent installing gems which depend on native extensions also missing from that image.

Since we switched to using a pre-built mainline image, all of the necessary dependencies should be there and we can remove this step to save time.

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This is hard to test without access to the images on ECR, but I can imagine a possible race-condition where a dependency is changed in the gemfile and a docs-preview build is triggered before that image can reach the cache. It should be rare though, and a rebuild should fix it.