docs: rm sidebar (JA) for Server 4.0

[kelvintaywl]

Description

As per our move to archive Server 4.0 content (see PR here), the Server 4.0 documents have been archived. However, our sidebar (when viewing in JA 🇯🇵 mode) still shows the Server 4.0 section, and these links go 404.

Since we did remove the section for EN here, this PR does the same by removing the JA section.

Reasons

For consistency, and to avoid confusion for our JA readers.

Content Checklist

Please follow our style when contributing to CircleCI docs. Our style guide is here: https://circleci.com/docs/style/style-guide-overview.

Please take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs) 😸:

• [ ] Break up walls of text by adding paragraph breaks.
• [ ] Consider if the content could benefit from more structure, such as lists or tables, to make it easier to consume.
• [ ] Keep the title between 20 and 70 characters.
• [ ] Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
• [ ] Check all headings h1-h6 are in sentence case (only first letter is capitalized).
• [ ] Is there a "Next steps" section at the end of the page giving the reader a clear path to what to read next?
• [ ] Include relevant backlinks to other CircleCI docs/pages.