[java-generator] Remove unused generation options

[andreaTP]

Is your enhancement related to a problem? Please describe

When we started the java-generator module we attempted to get an output closer to what the current "extensions" are producing.

After a few trials and fails, we have decided to drop this requirement to concentrate on the correctness and idiomaticity of the generated code.

There are in the codebase a few options that, likely, no one is using:

• prefix
• suffix
• always preserve unknown fields
• code structure

Before getting the java-generator out of the preview state, we want to clean up those to remove the maintenance burden from the codebase.

Describe the solution you'd like

Remove the above-mentioned options from the java-generator codebase keeping only the current "default" implementation.

Describe alternatives you've considered

Happy to hear if any user is using the mentioned flag and how they are doing so.

Additional context

No response