[Feature flag] Remove jira_issue_association_on_merge_request

Problem to solve

Remove jira_issue_association_on_merge_request feature flag added in &4637.

Rollout issue: #300271 (closed)

Tasks

• Remove feature flag and add changelog entry
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel

Rollback Steps

• This feature can be disabled by running the following Chatops command:

/chatops run feature set jira_issue_association_on_merge_request false