Remove Slack attachment in new issue presenter

What does this MR do?

Closes #324600 (closed)

The new issue Slack slash command /gitlab project-alias issue new title description currently returns a Slack attachment that includes assignee, milestone, labels and weight. Because we can't set these additional parameters in the attachment when creating the issue via Slash commands, why show them at all?

This MR:

• Removes the fields passed into the presenter so GitLab no longer sends an attachment back to Slack
• Changes the issue number in the text to a link that goes to the actual issue that was created (since this used to be in the attachment)

Screenshots (strongly suggested)

Original behaviour:

original-behaviour

New behaviour:

new-behaviour

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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

Related to #324600 (closed)