refactor: remove OpenAPI spec from ci

[LynxLynxx]

Description

Due to maintenance problems related to code generation and it's limitations frontend team decided to remove code generation from OpenAPI spec.

Related Issue(s)

Resolves input-output-hk/catalyst-voices#1128

Description of Changes

Remove flutter-ci+OPENAPI_CODE_GEN and it's unused depdendencies.

Related Pull Requests

Relates input-output-hk/catalyst-voices#1130

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• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module