Remove confusing placeholder for JIRA transition_id

What does this MR do?

It removes placeholder for transition_id in JIRA service integration.

Why was this MR needed?

The placeholder (2) is there even when user creates and enables the service and it is really confusing as it seems the transition_id was set to 2 although it is '' in the db.

Screenshots (if relevant)

=> #<JiraService:0x007f801c78a1d8
 id: 14,
 type: "JiraService",
 title: nil,
 project_id: 1500025,
 created_at: Mon, 03 Apr 2017 17:45:32 UTC +00:00,
 updated_at: Mon, 03 Apr 2017 17:45:32 UTC +00:00,
 active: true,
 properties: {"jira_issue_transition_id"=>"", 

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28574 (closed)