[6.0] Deprecate unused phpmailer language string

[brianteeman]

Pull Request for Issue # .

Summary of Changes

This string has never existed in phpmaailer and the phpmailer code stopped referencing it in 2013 PHPMailer/PHPMailer#18

Testing Instructions

code review only

Actual result BEFORE applying this Pull Request

Expected result AFTER applying this Pull Request

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[wojsmol]

I have tested this item ✅ successfully on 1f57e95

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[ceford]

I have tested this item ✅ successfully on 1f57e95

Could not find any instances of the deprecated strings other than those in the three .ini files.

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[richard67]

RTC

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[Bodge-IT]

Thanks @brianteeman and @ceford & @wojsmol for tests