From May 15, 2014 you will no longer be able to run polls within LinkedIn groups for any kind of survey. All old polls will be removed will no longer be accessable.
Members are encouraged to post their survey questions within groups and other members are expected to respond.
LinkedIn explains:
You can continue to engage with fellow members by posting a question to get the group’s response or sharing an update or participating in conversations directly on the LinkedIn homepage.
LinkedIn Polls in Groups will be retired on May 15, 2014.
Source: LinkedIn