Revert custom emoji rendering support across Issuables

What does this MR do and why?

Revert "Merge branch 'kp-show-custom-emoji-issuable-title' into 'master'"

This reverts merge request !129453 (merged) because of potential security risk outlined in https://gitlab.slack.com/archives/CETG54GQ0/p1695213839544489

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