Do not add schema / class name mapping where custom mapping exists

[wing328]

• fix Spring generates duplicated JsonSubTypes #13150 by cherry picking fix #13150 Do not add schema / class name mapping where custom mapping exists #13815
• to close [BUG][JAVA] AllOf with mapping generates wrong JsonSubTypes annotations #14917
• avoid unnecessary import

credits: @bernie-schelberg-mywave

cc @OpenAPITools/generator-core-team

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[WannabeSoftwareEngineer]

Hey @wing328,
thank you for all your great work. Is this pr expected to make to 6.5.0?

[welshm]

Change looks good to me for Spring/Java

Not familiar enough with the other generators, but looks good from what I do understand