Remove whitespace in sidebar when iterations disabled

What does this MR do and why?

Remove extra space under 'Milestones' in right issue sidebar when Iterations feature is not available (feature flag disabled or on FOSS).

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Disable the group_iterations feature flag

   Feature.enable(:group_iterations)

2. Load an issue
3. Look at right sidebar spacing

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.