[BUG] Generation creates redundant allOf objects

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

See existing tests:
https://github.com/OpenAPITools/openapi-generator/pull/14459/files#r1073301024

Generated source code generates redundant AllOf files despite models already having those properties inlined.
Generate file with suffix AllOf is not used.

openapi-generator version

master

OpenAPI declaration file content or url

See example in PR

Generation Details

See example in PR

Steps to reproduce

Run tests

Related issues/PRs

#14121 #2178 #14459 #2360 #14278

Suggest a fix

Short term:
Remove file

Long term:
I think inlining models introduces a lot of bugs in the generator and are inconsistently handled across different generators.
Proper approach would be to navigate architecture of the generator to use rely on normalization rules (and enable others to write ones themselves) but then keeping generation logic free of complex (and bug free transformations) like inlining models - that puts extra efforts on communicating changes across different generators etc.

Start with the existing tests and examples referenced in PR #14459, then run the generation tests described in the issue. Confirm that redundant AllOf files are produced while their properties are already inlined and that the generated files are unused; the issue also points to related issues #14121, #2178, #2360, and #14278 for context.