vtorc: cleanup database_instance location fields

[timvaillancourt]

Description

This PR cleans up location-related database_instance fields in VTOrc, replacing 3 x fields with a new field: cell varchar(32)

It seems none of the removed fields were actually used in logic, with physical_environment and region always undef/"" and data_center == the tablet cell

The 3 x fields are replaced by the new cell field. It isn't used in any logic yet - for now it is informational

Related Issue(s)

Resolves #18344

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.49%. Comparing base (2520f7f) to head (67c30c6).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/inst/instance_dao.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18339   +/-   ##
=======================================
  Coverage   67.49%   67.49%           
=======================================
  Files        1603     1603           
  Lines      262428   262433    +5     
=======================================
+ Hits       177114   177123    +9     
+ Misses      85314    85310    -4     

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[GuptaManan100]

LGTM!