OfflineMapAreasView: refactor and remove `Backported`

[yo1995]

#1145 removed the backported part of the view, so it is no longer needed. The changes were at https://github.com/Esri/arcgis-maps-sdk-swift-toolkit/pull/1145/files#diff-8396a89243d9464c71a7916b6f30deaa9a483c897ce1b69cc8f1e3ff9d6c6553