Clean up JiraService

Problems

• JiraService never works when it's executed as a hook (see #24940 (closed)), we always call it directly
• execute method in project services is meant to be executed by project hooks
• we use JiraService#supported_events to render correct options for configuring JIRA integration ("Trigger") in service settings screen, but we do not really support these events as project hooks

Proposal

• JiraService#execute should be a no-op
• override Service#test in JiraService so it calls JiraService#test_settings instead of JiraService#execute
• extract a separate method from JiraService#execute which will be called from Issues::CloseService#close_issue

This will fix #24777 (closed) and #24948 (closed).

/cc @smcgivern @felipe_artur