remove: eliminate legacy thumbnail creation plugin and associated code

CHANGELOG.md

ckanext/odsh/pdf_to_thumbnail/__init__.py

ckanext/odsh/pdf_to_thumbnail/action.py

ckanext/odsh/pdf_to_thumbnail/helpers.py

ckanext/odsh/pdf_to_thumbnail/plugin.py

ckanext/odsh/pdf_to_thumbnail/thumbnail.py

ckanext/odsh/plugin.py

ckanext/odsh/templates/snippets/package_item.html

ckanext/odsh/tools.py

setup.py