Remove thumbnail generation from process

arkindex/documents/tests/test_retrieve_elements.py

arkindex/process/builder.py

...ess/migrations/0032_remove_process_generate_thumbnails.py

arkindex/process/models.py

arkindex/process/serializers/imports.py

arkindex/process/tests/test_create_process.py

arkindex/process/tests/test_processes.py

arkindex/sql_validation/process_elements_filter_ml_class.sql

arkindex/sql_validation/process_elements_filter_type.sql

arkindex/sql_validation/process_elements_top_level.sql

arkindex/sql_validation/process_elements_with_image.sql