[4.0] Delete unused strings

[Quy]

Testing Instructions

Code review.
Confirm that string constants are not being used.

[chmst]

I have tested this item ✅ successfully on 85bfc84

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[himanshu007-creator]

I have tested this item ✅ successfully on 85bfc84

These 2 String constants are not being used anywhere in code

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/33974.}

[richard67]

RTC

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[wilsonge]

Thanks!