vtorc: Clean up more unused code

[dbussink]

This goes a bit further than previous just deadcode cleanup, but it digs deeper into which parts are actually unused. We don't seem to use any of the binary log file positioning logic, since we all use GTID based logic so that can all be cleaned up.

This then removes a bunch of other unused fields etc. as well.

Lastly, we rename the data_center attribute to cell since that's the internal Vitess terminology.

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[codecov]

Codecov Report

Attention: Patch coverage is 57.50000% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 68.06%. Comparing base (e55897b) to head (2b14f17).

Files	Patch %	Lines
go/vt/vtorc/inst/instance_dao.go	16.66%	15 Missing ⚠️
go/vt/vtorc/inst/analysis_dao.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15609      +/-   ##
==========================================
- Coverage   68.09%   68.06%   -0.03%     
==========================================
  Files        1561     1559       -2     
  Lines      195288   195016     -272     
==========================================
- Hits       132974   132732     -242     
+ Misses      62314    62284      -30     

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[GuptaManan100]

Closing since it is being superseded by #15617.

[deepthi]

We don't seem to use any of the binary log file positioning logic, since we all use GTID based logic so that can all be cleaned up.

For future reference, this wasn't accurate. We do need to keep the binlog related fields around in the database.