CSR bot should not remove CSR label if one of the CSR issues is withdrawn

While testing SKARA-1714, I discovered a bug in the PullRequestWorkItem of the CSR bot.

Currently, the CSR bot would remove CSR label if all the CSR issues of the pr is closed(whether approved or withdrawn). However, if the CSR label is added via using '/csr needed' command and the pr contains withdrawn CSR issues, the CSR should not remove the CSR label.