Remove add issues modal code from issue boards [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This feature has been disabled since 13.6 with a feature flag. Seems like we have not gotten any feedback that we should enable it, so let's remove it from the code base.

One motivation for removing it: It uses the Vue runtime template compiler. Removing this will bring us one step closer to shipping Vue without the template compiler which will bring a performance improvement across all pages.

• Closes #205169 (closed)
• Closes #292803 (closed)

Screenshots (strongly suggested)

N/A

Does this MR meet the acceptance criteria?

Conformity

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