fix eclipse warnings

[rzpt]

Only generated _plainSerDe when it is used.

Partial fix for #1635

Before this PR

Generated code has a lot of warnings in eclipse.

After this PR

==COMMIT_MSG== Fix unused warnings in eclipse ==COMMIT_MSG==

Possible downsides?

Untested in intellij since I don't use it. Only tested on a single internal repo.

[changelog-app[bot]]

Generate changelog in changelog/@unreleased

Type

• [ ] Feature
• [x] Improvement
• [ ] Fix
• [ ] Break
• [ ] Deprecation
• [ ] Manual task
• [ ] Migration

Description Fix warnings in eclipse

Check the box to generate changelog(s)

• [x] Generate changelog entry