Remove unneeded loading of the MySQL driver

[dbussink]

These cases probably at some point actually used MySQL with Go's stdlib database/sql logic, but no longer do.

This is mostly testing logic except for vtorc. The initial code port from Orchestrator to vtorc likely kept the logic for a MySQL backend but we've long since only had vtorc using sqlite internally.

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[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.66%. Comparing base (696fe0e) to head (56a749b).
Report is 117 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15502      +/-   ##
==========================================
- Coverage   67.41%   65.66%   -1.76%     
==========================================
  Files        1560     1563       +3     
  Lines      192752   194474    +1722     
==========================================
- Hits       129952   127707    -2245     
- Misses      62800    66767    +3967     

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[dbussink]

Load it here since here is also where we do sql.Open.

[GuptaManan100]

LGTM