Fix NPE on synchronous repository export (#2040)

When a repository was exported synchronously with metadata,
the server ran into a NullPointerException. The was because
the resource tried to write an export result to a stored
export item, that does not exist for synchronous exports.

This simply removes this call.

gradle/changelog/npe_on_export.yaml

@@ -0,0 +1,2 @@
+- type: fixed
+  description: NPE on synchronous repository export ([#2040](https://github.com/scm-manager/scm-manager/pull/2040))

scm-webapp/src/main/java/sonia/scm/api/v2/resources/RepositoryExportResource.java

@@ -457,8 +457,6 @@ public class RepositoryExportResource {
       return Response.status(204).build();
     } else {
       StreamingOutput output = os -> fullScmRepositoryExporter.export(repository, os, password);
-      exportService.setExportFinished(repository);
-
       return Response
         .ok(output, "application/x-gzip")
         .header("content-disposition", createContentDispositionHeaderValue(repository, fileExtension))