Hide warning to disable GitLab issues in Jira integration form

What does this MR do?

Issue #228824 (closed).

When GitLab issues are already disabled, we can hide the warning to disable GitLab issues in Jira integration form. The copy to be hidden is as follows:

Displaying Jira issues while leaving the GitLab issue functionality enabled might be confusing. Consider disabling GitLab issues if they won’t otherwise be used.

Screenshots

GitLab Issues enabled	GitLab Issues disabled

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team