[Unfork] Reset repository picker options after submission

Description

Currently at Unfork, after deleting repositories, those repositories stay as options.

The RepositoryPicker component should reset options after user has submitted the repositories for deletion.

For TopicSpace, i think it is ok to leave the repositories selected there after users updated the topics as they might wanna add more topics to the same selected topics.

Relevant code

• unfork/index.tsx -> Probably just need to reset the selectedItems state here.

But there may be unintended UX behaviors caused by downshift component after resetting the state, so please play around with the dropdown after making the changes to ensure that the component still work as intended.