Remove superfluous HTML length validation refs

What does this MR do and why?

Remove superfluous HTML length validation references. The removed code is unreachable and not used anymore. The documentation is referencing unused limitations.

• Remove unused constant

• Remove superfluous mentions in documentation

• Refs: !163982 (comment 2077743402) !140857 (diffs)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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How to set up and validate locally

1. Search for uses of const in code e.g. ag TITLE_HTML_LENGTH_MAX 1.1 should come up empty