Rm the use of mixins from issue_card _inner

What does this MR do?

As our codebase grows larger it seems like the general consensus as a group is to remove mixins as a vue component pattern.

As a result @psimyn and I decided to resurrect the RFC gitlab-org/frontend/rfcs#32 (closed) to start looking at ways to share code in components between EE and CE components. This is obviously a work in progress but a step in what we feel to be in the right direction.

This component actually didnt need to use mixins and the functions it used didnt exist (which is a risk when we dont know where our functions are being defined).

Screenshots

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