Remove variable declaration that is unused

What does this MR do?

• The unused chef-base-be-sidekiq-role isn't being utilized and therefore can be removed
• This will enable us to remove those roles from our chef server

Addresses: gitlab-com/gl-infra/scalability#1152 (closed)

Author Check-list

Please read the Contributing document and once you do, complete the following:

• Assign to the correct reviewer per the contributing document
• Apply the correct metadata per the contributing document
• Link to related MRs for applying the changes on other environments
• Link to related Chef changes
• If necessary link to a Criticality 4 Change Request issue

Reviewer Check-list

• Reviewed the diff jobs to confirm changes are as expected
• No changes shown in the diffs not associated with this MR - This may require a rebase or further investigation