BulkImports: Remove unnecessary transform from GroupAvatar

What does this MR do?

Related to: #333300 (closed)

The BulkImports feature (https://docs.gitlab.com/ee/development/bulk_import.html) uses ETL pipelines. When the GroupAvatarPipeline was introduced (!62714 (merged)) it added an empty Transformation, which is not required. This MR removes the unnecessary transform method and simplify GroupAvatarPipeline tests.

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