Just clean that place up. The only spiders you need to worry about are spider mites which arent even realy spiders. Regular spiders wont hurt your plants at all if anything they will help keep other bugs away. If you get rid of their webs though and start using the shed regularly they wont come back. Definetly make sure no holes for light to escape. Wouldnt be a bad idea to bomb either even though its not that necessary for the spiders your talking about it might help keep away other bugs that will harm you plants later.