Remove useless docs build on production branch

What does this MR do?

Removes build:docs from production branch. docs are only deployed on master.

How confident are you it won't break things if deployed?

Sure

How to test

Not possible

Checklist

• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• added to the next milestone (see https://gitlab.com/foodsharing-dev/foodsharing/-/milestones, unless it has a "for:Dev" label)
• added an entry to CHANGELOG.md