dev/core#1921 remove isoToMysql

dev/core#1921 remove isoToMysql

Overview
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Remove unnecessary isoToDate function

Before
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Code compatible with packages code of 6 years ago

After
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will break if you revert 6 year old fix....

Technical Details
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The test testMailerStats passes through this line of code - which
I think is probably about the most we can confirm testing wise as we expect there
to be no reason to cast to mysql anymore

Comments
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https://lab.civicrm.org/dev/core/-/issues/1921