refactor: Remove dead code from MySQLi Connection related to PHP 5

[kamil-tekiela]

Description
Removed old code that had a condition clause applicable only to old PHP 5 version.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[mergeable]

Hi there, kamil-tekiela! 👋

Thank you for sending this PR!

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Sincerely, the mergeable bot 🤖

[michalsn]

Thank you.