Mapper: Drop @ExportName

[ljacqu]

Drop support for @ExportName, which is an annotation set on the setter or getter method of a bean property to give it a different name.

Do you use this? Please comment in this issue if you want me to reconsider the removal.

[ljacqu]

We keep it: initially, there was an assumption that this feature would cause a lot of issues, but this turned out to not be true -> fine to keep