Remove unused slot variables from user_callout_dismisser

What does this MR do and why?

Remove unused slot variables from user_callout_dismisser.vue.
The following slot variables were removed:

• isAnonUser
• isDismissed
• isLoadingMutation
• isLoadingQuery
• mutationError
• queryError

Only dismiss and shouldShowCallout remain, as they are the only ones actually used in the codebase.

Closes Issue #577584

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References

• Related MR: !169515 (merged) (from the following discussion)

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Screenshots or screen recordings

No UI changes.

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How to set up and validate locally

Run the specific test to verify the changes and ensure all tests pass (should show 11 tests passing):

yarn jest spec/frontend/vue_shared/components/user_callout_dismisser_spec.js --verbose