Clean up duplicate datetime parsing code

[dbussink]

This cleans up some old legacy code which predates our much improved parsing capability with the datetime package. Let's use that and remove the now unused code from sqltypes.

The conversion is only used in the driver, so move it there then and use datetime directly.

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[codecov]

Codecov Report

Attention: Patch coverage is 78.37838% with 8 lines in your changes missing coverage. Please review.

Project coverage is 67.70%. Comparing base (10ff5e3) to head (06810e5).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vitessdriver/convert.go	78.37%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17582      +/-   ##
==========================================
- Coverage   67.70%   67.70%   -0.01%     
==========================================
  Files        1585     1585              
  Lines      254833   254827       -6     
==========================================
- Hits       172547   172539       -8     
- Misses      82286    82288       +2     

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