Use Action Cable to update notes in real-time

What does this MR do and why?

Removes the action_cable_notes feature flag since this has been enabled on GitLab.com for more than 2 weeks now.

Instead of polling the notes endpoint regularly, we use Action Cable to signal that a change has been made and then we request the updated notes. This results in more immediate updates.

Related to #412823 (closed)

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.

Related to #412823 (closed)