Product category image should only be displayed in the image column in the admin, rather than all custom columns.
Streamline wpsc_custom_category_column_data() and adhere to coding standards.
Cleanup formatting and add documentation for wpsc_custom_category_column_data().
Remove unused global $wpdb in wpsc_custom_category_column_data().
Merge pull request #1421 from benhuson/PR_admin_cat_image_col

Fix product category image displaying in other custom columns in the admin.